Aberration compensating optical element, optical system, optical pickup device, recorder and reproducer

ABSTRACT

An aberration compensating optical element includes: a diffractive structure having a plurality of ring-shaped zone steps formed into substantially concentric circles on at least one surface of the aberration compensating optical element; wherein the aberration compensating optical element is adapted for being disposed on an optical path between a light source for emitting a light having a wavelength of not more than 550 nm, and an objective lens made of a material having an Abbe constant of not more than 95.0 at a d-line; and wherein the following inequality is satisfied: P λ1 &lt;P λ0 &lt;P λ2 , where P λ0  is a paraxial power (mm −1 ) of the aberration compensating optical element at the wavelength λ 0  of the light emitted from the light source; P λ1  is a paraxial power (mm −1 ) of the aberration compensating optical element at a wavelength λ 1  which is 10 nm shorter than the wavelength λ 0 ; and P λ2  is a paraxial power (mm −1 ) of the aberration compensating optical element at a wavelength λ 2  which is 10 nm longer than the wavelength λ 0 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an aberration compensating opticalelement having a diffractive structure, an optical system comprisingthis aberration compensating optical element, an optical pickup device,a recorder and a reproducer. Further, the present invention relates toan aberration compensating optical element which is disposed on anoptical path between a light source and a high NA objective lens havingat least one plastic lens and which can minimize a change in a sphericalaberration of the objective lens, which is caused by a temperaturechange; an optical system comprising this aberration compensatingoptical element and a high NA objective lens having at least one plasticlens, wherein the optical system is used for an optical pickup devicefor carrying out at least one of the record of information on an opticalinformation recording medium and the reproduction of information from anoptical information recording medium; an optical pickup devicecomprising this optical system, for carrying out at least one of therecord of information on an optical information recording medium and thereproduction of information from an optical information recordingmedium; a recorder comprising this optical pickup device, for carryingout the record of at least one of a sound and an image on an opticalinformation recording medium; and a reproducer for carrying out thereproduction of at least one of a sound and an image from an opticalinformation recording medium.

2. Description of Related Art

In recent years, a new high density storage optical pickup system usinga blue-violet semiconductor laser light source having an oscillationwavelength λ of about 400 nm and an objective lens having an enhancednumerical aperture (NA) of about 0.85, has been researched anddeveloped. On an optical disk having a diameter of 12 cm, which is thesame as that of a DVD (NA=0.6, λ=650 nm, storage capacity 4.7 GB), forexample, an optical pickup system (NA=0.85, λ=400 nm) can record 25 GBof information.

However, when such a high NA objective lens and a short wavelength lightsource having the oscillation length of about 400 nm are used, a problemthat an axial chromatic aberration occurs at an objective lens, iscaused. In general, a laser light emitted from a semiconductor laser hasa single wavelength (single mode), and it is thought that the axialchromatic aberration does not occur. In practice, there is somepossibility that the mode hopping in which a center wavelength of alaser light is instantly hopped about several nanometers by changing thetemperature, the output of the light or the like, occurs. Because themode hopping is a phenomenon that a wavelength is changed so instantlythat the focusing of an objective lens cannot follow the wavelengthchange, if a chromatic aberration occurring due to the objective lens isnot compensated, a defocus component caused by the mode hopping isadded. As a result, a wavefront aberration increases. In case of using ahigh NA objective lens or a short wavelength light source, the wavefrontaberration caused at the mode hopping specially increases for thefollowing reason. When due to a wavelength change Δλ, the sphericalaberration is not changed at the objective lens and the back focus fb ischanged by Δfb, if the objective lens is focused in an optical axisdirection against a change of the back focus, the root mean square valueW_(rms) of the wavefront aberration is 0. When the objective lens is notfocused, the value W_(rms) is expressed by the following Formula (1).

W _(rms)=0.145·{(NA)² /λ}/|Δfb|  (1)

For example, when an optical pickup system for DVD (NA=0.6, λ=650 nm) iscompared with one for an optical disk (NA=0.85, λ=400 nm), in case ofthe same Δfb, the wavefront aberration occurring in the latter opticalpickup system increases by 3.26 times. That is, if the permissible valueof the wavefront aberration is the same in both systems, the permissiblevalue of |Δfb| decreases by 1/3.26 in the latter system. Therefore, itis required that the axial chromatic aberration on a wave surface of alight which transmits through the objective lens and is condensed on astorage surface of an optical disk, should be small.

In such a high density optical pickup system, in order to save the costthereof and to lighten it, it is desired that an objective lens is aplastic lens like a conventional CD system and a DVD system. However, ina high NA plastic objective lens, when the temperature changes, thechange in the spherical aberration, which is caused by the refractiveindex change of plastic, is large because it increases in proportion tothe fourth power of NA. Therefore, in practical use, the above changebecomes a significant problem.

As a cemented doublet type of objective lens for optical disk, which isformed so as to compensate the chromatic aberration, ones disclosed inJapanese Patent Publications (laid-open) No. Tokukai-Sho 61-3110 and No.Tokukai-Sho 62-286009, are known. The above lens which is formed bycombining a lens made of low dispersion material having a positiverefractive power and a lens made of high dispersion material having anegative refractive power, is unsuitable for an objective lens used fora optical disk, which is required to be light. Because there is a limitof the dispersion of the material, the above lens itself becomes heavyin order to obtain a high NA and to increase the refractive power ofeach lens.

In Japanese Patent Publication (laid-open) No. Tokukai-Hei 11-174318, anobjective lens having a doublet lens structure, wherein the numericalaperture on an optical disk side is 0.85, and a hologram is provided onan optical surface in order to compensate the axial chromaticaberration, is disclosed. However, when the hologram comprises aring-shaped zone structure having a plurality of fine steps which areformed into concentric circles, in a high objective lens which has atendency to decrease a curvature of the optical surface, a shadow of thering-shaped zone structure largely influences the transmitted light andthen the transmittance of the light decreases. Therefore, the aboveobjective lens is unsuitable for a high density storage optical pickupsystem as an information writing system which is required to have a highlight utilization efficiency.

As an aberration compensating optical element for compensating the axialchromatic aberration of the objective lens, one disclosed in JapanesePatent Publication (laid-open) No. Tokukai-Hei 6-82725, is known. Whenthe aberration compensating optical element in which a plurality ofsteps are formed as ring-shaped zones having the form of concentriccircles around an optical axis on a plane surface perpendicular to theoptical axis, is disposed in parallel light flux, a reflected light in adiffractive structure returns in the same direction as an incidentlight. As a result, a ghost signal is generated in a detection system ofthe optical pickup device. Therefore, The above lens is unsuitable foran aberration compensating optical element used in an optical pickupsystem.

SUMMARY OF THE INVENTION

In order to solve the above problem, an object of the present inventionis to provide an optical system for optical pickup device, which cancompensate the axial chromatic aberration with a relatively simplestructure even though a light source having a bad monochromaticity or alight source in which a wavelength of the light suddenly changes, isused in a high density storage optical pickup system or the like, andwhich can be manufactured in a low cost; an optical pickup devicecomprising the above optical system; and a recorder and a reproducer,comprising the above optical pickup device.

Another object is to provide an aberration compensating optical elementwhich can compensate the spherical aberration and the sine conditionbecause of a large numerical aperture on the optical informationrecording medium side, and can compensate the axial chromatic aberrationwhen an objective lens in which the axial chromatic aberration remains,is used in order to downsize an optical device, to thin it, to lightenit, and to decrease the cost thereof.

Further, another object is to provide an optical system used in opticalpickup device, for suppressing the change in the spherical aberration,which is caused at an objective lens by the temperature change, with arelatively simple structure, even though a high NA plastic objectivelens is used in, for example, a high density storage optical pickupsystem; an optical pickup device comprising the above optical system;and a recorder and a reproducer, comprising the above optical pickupdevice.

Further, another object is to provide an aberration compensating opticalelement which can compensate the change in the spherical aberration,which is caused at an objective lens by the temperature change, when aplastic objective lens in which the change of the spherical aberrationis large because of the temperature change, is used.

In order to accomplish the above-mentioned object, in accordance withthe first aspect of the present invention, an aberration compensatingoptical element comprises:

a diffractive structure having a plurality of ring-shaped zone stepsformed on at least one surface of the aberration compensating opticalelement;

wherein the aberration compensating optical element is adapted for beingdisposed on an optical path between a light source for emitting a lighthaving a wavelength of not more than 550 nm, and an objective lens madeof a material having an Abbe constant of not more than 95.0 at a d-line;and

wherein the following formula (2) is satisfied:

P_(λ1)<P_(λ0)<P_(λ2)  (2),

where P_(λ0) is a paraxial power (mm⁻¹) of the aberration compensatingoptical element at the wavelength λ₀ of the light emitted from the lightsource;

P_(λ1) is a paraxial power (mm⁻¹) of the aberration compensating opticalelement at a wavelength λ₁ which is 10 nm shorter than the wavelengthλ₀; and

P_(λ2) is a paraxial power (mm⁻¹) of the aberration compensating opticalelement at a wavelength λ₂ which is 10 nm longer than the wavelength λ₀.

The above formula (2) is a condition of the paraxial power of theaberration compensating optical element disposed between this lightsource emitting a light having a wavelength of not more than 550 nm andthe objective lens in which the axial chromatic aberration remains, tocompensate the axial chromatic aberration occurring at the objectivelens, in an optical pickup device in which the light source having a badmonochromaticity or a light source in which a wavelength of the lightsuddenly changes, is used. The formula (2) has a meaning that the axialchromatic aberration caused in the whole system of the optical pickupsystem including the aberration compensating optical element and theobjective lens is compensated by increasing the paraxial power of theaberration compensating optical element at the wavelength λ₂ which is 10nm longer than the wavelength λ₀ of the light emitted from the lightsource, in order to over-compensate the axial chromatic aberrationcaused at the aberration compensating optical element, and by cancelingthe under-corrected axial chromatic aberration caused by the objectivelens at the wavelength λ₂. By combining the aberration compensatingoptical element according to the present invention and the objectivelens, even though the axial chromatic aberration caused at the objectivelens is not precisely compensated, it is possible to use the objectivelens causing a problem relating to the remaining axial chromaticaberration thereof, in an optical pickup device in which a shortwavelength light source having a bad monochromaticity is provided. Theparaxial power of the aberration compensating optical element is thepower of the whole system of the aberration compensating opticalelement, which is calculated by combining the refractive power as arefractive lens and the diffractive power generated by only adiffractive structure.

In accordance with the second aspect of the present invention, anaberration compensating optical element comprises:

a diffractive structure having a plurality of ring-shaped zone stepsformed on at least one surface of the aberration compensating opticalelement;

wherein the aberration compensating optical element is adapted for beingdisposed on an optical path between a light source for emitting a lighthaving a wavelength of not more than 550 nm, and an objective lens madeof a material having an Abbe constant of not more than 95.0 at a d-line;and

wherein at least one ring-shaped zone step having a step distance Δ (mm)in a direction of an optical axis between adjacent steps of theplurality of ring-shaped zone steps is formed within an effectivediameter so that m, defined by following equations:

m=INT(Y),

Y=Δ×(n−1)/(λ₀×10⁻³)  (3),

 is an integer except 0 and ±1,

where INT(Y) is an integer obtained by rounding Y, λ₀ is the wavelength(nm) of the light emitted from the light source, and n is a refractiveindex of the aberration compensating optical element at the wavelengthλ₀ (nm).

The above formula (3) has a meaning that the difference Δ in a directionof an optical axis between adjacent steps of the plurality ofring-shaped zone steps of the aberration compensating optical element isdetermined so that a diffracted light amount of the higher orderdiffracted light having a diffraction order which is two or more, islarger than those of the diffracted lights having the other diffractionorders, when the incident light flux is diffracted by the diffractivestructure formed on the optical surface of the aberration compensatingoptical element.

In accordance with the third aspect of the present invention, anaberration compensating optical element comprises:

a diffractive structure having a plurality of ring-shaped zone stepsformed on at least two surfaces of the aberration compensating opticalelement;

wherein the aberration compensating optical element is adapted for beingdisposed on an optical path between a light source for emitting a lighthaving a wavelength of not more than 550 nm, and an objective lens madeof a material having an Abbe constant of not more than 95.0 at a d-line.

In a general optical material, when the wavelength of the light becomesshort, the change in the refractive index, which is caused by a slightwavelength change, becomes large. In case of using a short wavelengthlight source emitting a light having a wavelength λ of not more than 550nm, when a slight wavelength change is caused, the axial chromaticaberration caused at the objective lens becomes large. Therefore, therequired power of the aberration compensating optical element as adiffractive lens must be large in order to compensate the axialchromatic aberration caused at the objective lens. When the power of theobjective lens is φ_(OBJ), and the power of the aberration compensatingoptical element is φ_(SA), in order not to change the back focus by thewavelength change in the compound system including the objective lensand the aberration compensating objective element, the following formula(4) may be satisfied:

dφ _(SA) /dλ=−dφ _(OBJ) /dλ  (4).

On the other hand, the relation between the change in the power of theobjective lens, which is caused by the wavelength change, and the changein the back focus, is expressed by the following formula (5). Becausethe power of the aberration compensating optical element as adiffractive lens is proportional to the wavelength, the power isexpressed by the following formula (6).

dφ _(OBJ) /dλ=−(dfB/dλ)·dφ _(OBJ) ²  (5)

dφ _(SA) /dλ=φ _(SA)/λ  (6)

When the formulas (5) and (6) are substituted for the formula (4), thepower φ_(SA) of the aberration compensating optical element as adiffractive lens is expressed by the following formula (7):

φ_(SA)=(dfB/dλ)·dφ _(OBJ) ²  (7).

For example, in case of a general objective lens for DVD, which has afocal length of 3.33 mm, a working wavelength of 650 nm, an NA of 0.6,an entrance pupil with a diameter φ of 4 mm, and a νd of 55, becausedfB/dλ is 0.15 μm/nm, the power φ_(SA) of the aberration compensatingoptical element is determined as shown in the following formula (8):

φ_(SA)=0.15×10⁻³·650·(1/3.33)²=1/92.3 (mm ⁻¹)=1.1×10⁻² (mm ⁻¹)  (8).

In case of the objective lens for a high density storage optical pickupdevice, which has a focal length of 2.35 mm, a working wavelength of 405mm, an NA of 0.85, an entrance pupil with a diameter φ of 4 mm, and a νdof 55, because dfB/dλ is 0.40 μm/nm, the power φ_(SA) of the aberrationcompensating optical element is determined as shown in the followingformula (9):

φ_(SA)=0.40×10⁻³·405·(1/2.35)²=1/34.1 (mm ⁻¹)=2.9×10⁻² (mm ⁻¹)  (9).

That is, it is required that the aberration compensating optical elementfor a high density storage optical pickup device has a power which is2.7 times larger than one for DVD. In practical, because an objectivelens for a high density storage optical pickup device has a large NA, afocal depth is small. Therefore, it is required that the axial chromaticaberration is precisely compensated. The required power of theaberration compensating optical element for a high density storageoptical pickup device becomes larger than the formula (9).

The optical path difference Φ generated in a transmitted wave surface bythe aberration compensating optical element having the power φ_(SA) isexpressed by the following formula (10) as a function of the height hfrom the optical axis,

Φ=(φ_(SA)/2)·h ²  (10).

The interval Λ of the adjacent ring-shaped zones in the diffractivering-shaped zone structure formed on the aberration compensating opticalelement, which is measured in a direction perpendicular to the opticalaxis, is expressed by the following formula (11):

Λ=m·λ ₀/(dΦ/dh)  (11),

where λ₀ is the optimum wavelength, and m is the diffraction order ofthe diffracted light having the maximum diffracted light amount.

When the formula (11) is substituted for the formula (10), the intervalΛ of the adjacent ring-shaped zones in the diffractive ring-shaped zonestructure is determined by the following formula (12):

Λ=m·λ ₀/(φ_(SA) ·h)  (12).

Therefore, in the above-described aberration compensating opticalelement for DVD, when the optimum wavelength for the ring-shaped zonestructure is 650 nm, the interval Λ₆₅₀ of the diffractive ring-shapedzone structure in the height 2 mm from the optical axis, is expressed bythe following formula (13):

Λ₆₅₀ =m·650×10⁻³/(1/92.3·2)=30·m (μm)  (13).

On the other hand, in the above-described aberration compensatingoptical element for a high density storage optical pickup device, whenthe optimum wavelength for the ring-shaped zone structure is 405 nm, theinterval Λ₄₀₅ of the diffractive ring-shaped zone structure in theheight 2 mm from the optical axis, is expressed by the following formula(14):

Λ₄₀₅ =m·405×10⁻³/(1/34.1·2)=6.9·m (μm)  (14).

In the formula (14), when the diffractive ring-shaped zone structure isdetermined so that the first order diffracted light has the maximumdiffracted light amount, the interval of the adjacent ring-shaped zonesin the diffractive ring-shaped zone structure is 6.9 μm in the positioncorresponding to the entrance pupil of the objective lens. Therefore,there is some possibility that a light mount loss occurring due to aphase non-matching portion of the ring-shaped zone, which is caused bythe transferring the shape of edge portion of a diamond tool, when amold for injection molding is processed by cutting with SPDT, influencesgreatly. Further, in case that the interval of the ring-shaped zones inthe diffractive ring-shaped zone structure is small, it is difficultthat the shape of the ring-shaped zone is transferred in the molding.Thereby, a light amount loss occurring due to a phase non-matchingportion also influences greatly.

As described above, like the aberration compensating optical elementaccording to the second aspect of the present invention, the differenceΔ (mm) in a direction of an optical axis between adjacent steps of theplurality of ring-shaped zone steps of the aberration compensatingoptical element is determined so that a diffracted light amount of thehigher order diffracted light having a diffraction order which is m ormore (where m is an integer of not less than 2), is larger than those ofthe diffracted lights having the other diffraction orders. From theabove formula (14), the interval of the adjacent ring-shaped zones inthe diffractive ring-shaped zone structure can be m times wider.Therefore, the influence of a light amount loss caused by the phasenon-matching portion can be small.

Further, like the aberration compensating optical element according tothe third aspect of the present invention, when the diffractivering-shaped zone structures are formed on n (n is an integer of not lessthan 2) or more optical surface and the required power φ_(SA) of theaberration compensating optical element is equally divided into noptical surfaces, the power of each surface becomes φ_(SA)/n. From theformula (12), the interval of the ring-shapes zones in the diffractivering-shaped zone structure can be n times larger. Therefore, theinfluence of a light amount loss caused by the phase non-matchingportion can be small.

For example, the diffractive ring-shaped zone structures are formed ontwo optical surfaces. Further, the difference Δ (mm) in a direction ofan optical axis between adjacent steps of the plurality of ring-shapedzone steps of the aberration compensating optical element is determinedso that a diffracted light amount of the higher order diffracted lighthaving a diffraction order of 2, is larger than those of the diffractedlights having the other diffraction orders. In this case, from theformulas (12) and (14), the interval of the adjacent ring-shaped zonesin the diffractive ring-shaped zone structure can be 27.6 μm in theposition corresponding to the entrance pupil of the above objectivelens.

In accordance with the fourth aspect of the present invention, anaberration compensating optical element comprises:

a single lens;

wherein the single lens has one optical surface having a diffractivestructure having a plurality of ring-shaped zone steps formed on a planesurface and another optical surface opposite to the one optical surface,which has a concave refractive surface; and

wherein the aberration compensating optical element is adapted for beingdisposed on an optical path between a light source for emitting a lighthaving a wavelength of not more than 550 nm, and an objective lens madeof a material having an Abbe constant of not more than 95.0 at a d-line.

In the present invention, the phrase “a diffractive structure having aplurality of ring-shaped zone steps is formed on a plane surface” hasthe same meaning that a line (an envelope) by which the peaks of thering-shaped zone steps are connected with each other, becomes a straightline in FIGS. 2A and 12A described below.

When the diffractive ring-shaped zone structure is formed on a planesurface, the reflected light which is reflected by the diffractivestructure, travels in a different direction from the incident light.Therefore, it can be prevented that a ghost signal is generated in adetection system of the optical pickup device. Further, because thereflecting power of the optical surface on which the diffractivering-shaped zone structure is formed, is 0, the total power which is thesum of the refractive power and the diffractive power, can be calculatedby the formula (7). A refractive surface having a negative refractivepower is formed on an optical surface opposite to the optical surface onwhich the diffractive structure is formed, so that the absolute value ofthe refractive power of the refracting surface is same as that of thepower calculated by the formula (7). Thereby, the power of the wholesystem of the aberration compensating optical element can be 0.Therefore, it becomes easy that the aberration compensating opticalelement is disposed in the parallel light flux.

Further, when the diffractive ring-shaped zone structure is formed on aplane surface, it is possible that the aberration compensating opticalelement for a high density storage optical pickup device, in which theinterval of the ring-shaped zone in the diffraction ring-shapedstructure is several micro-meter as calculated by the above formula(14), can be prepared by using the electron beam drawing by which a finediffractive structure can be formed without the form error. The methodfor preparing the fine diffractive structure by using the electron beamdrawing is disclosed in “OPTICAL DESIGN, Journal of Optics Design Group,No. 20, 2000.2.25, pp.26-31”.

In the above-described aberration compensating optical element, thefollowing formula (15) may be satisfied:

0.5×10⁻² <P _(D)<15.0×10⁻²  (15),

where P_(D) is a paraxial power (mm⁻¹) of the diffractive structure andis defined by the following equation:

P _(D)=Σ(−2·b _(2i) ·ni),

when an optical path difference function is defined by the followingequation:

Φbi=ni·(b _(2i) ·hi ² +b _(4i) ·hi ⁴ +b _(6i) ·hi ⁶+ . . . ),

as a function that an optical path difference Φbi added to a wavefronttransmitting through the aberration compensating optical element, by thediffractive structure formed on an i-th surface of the aberrationcompensating optical element, is expressed by using a height hi (mm)from the optical axis; where ni is a diffraction order of a diffractedlight having a maximum diffracted light amount among a plurality ofdiffracted lights generated by the diffractive structure formed on thei-th surface, and b_(2i), b_(4i), b_(6i), . . . are a second ordercoefficient of the optical path difference function, a fourth order one,a sixth order one . . . , respectively.

As described above, for example, in order to compensate the axialchromatic aberration occurring at the objective lens having a focallength of 2.35 mm, a working wavelength of 405 mm, an NA of 0.85, anentrance pupil with a diameter φ of 4 mm, and a νd of 55, the aberrationcompensating optical element for a high density storage optical pickupdevice is required to have a diffractive power φ_(SA) of about 2.9×10⁻²(mm⁻¹). In practice, because an objective lens for a high densitystorage optical pickup device has a large NA, a focal depth is small.Therefore, it is required that the axial chromatic aberration isprecisely compensated. The required power of the aberration compensatingoptical element for a high density storage optical pickup device becomeslarger than the above value. Further, the required power of theaberration compensating optical element is changed according to thefocal length of the objective lens and the Abbe constant. Therefore,like the above formula (15), the condition is determined as a preferablerange of the power of the aberration compensating optical element for ahigh density storage optical pickup device.

At the lower limit of the formula (15), the axial chromatic aberrationof the wave surface condensed on the information recording surface ofthe optical information recording medium is not too under-corrected. Atthe upper limit of the formula (15), the axial chromatic aberration ofthe wave surface condensed on the information recording surface of theoptical information recording medium is not too over-corrected. In orderto obtain the above function, it is more preferable that the followingformula (16) is satisfied:

1.0×10⁻² <P _(D)<10.0×10⁻²  (16).

In the above-described aberration compensating optical element, it ispreferable that the paraxial power P_(λ0) of the aberration compensatingoptical element is substantially zero at the wavelength λ₀ of the lightemitted from the light source. Therefore, it becomes easy that theaberration compensating optical element is disposed in the parallellight flux. In the concrete, wherein the following formulas (17) to (19)are satisfied:

P_(D)>0  (17)

P_(R)<0  (18)

−0.9<P _(D) /P _(R)<−1.1  (19),

where P_(R) is a refractive power (mm⁻¹) of the aberration compensatingoptical element as a refractive lens.

In the above-described aberration compensating optical element, it ispreferable that the diffractive structure has such a sphericalaberration property that a spherical aberration of an emergent lightflux is changed in an under-corrected direction or an over-correcteddirection when a wavelength of an incident light flux is shifted to alonger wavelength side;

wherein the diffractive structure is formed so as to satisfy thefollowing inequality:

0.2≦|(P _(hf) /P _(hm))−2|≦6.0  (20),

where P_(hf) is a first interval in a direction to perpendicular to anoptical axis of the diffractive structure between adjacent steps of thering-shaped zones of the diffractive structure at a diameter hf which isa half of a maximum effective diameter hm, and P_(hm) is a secondinterval in the direction to perpendicular to the optical axis of thediffractive structure between adjacent steps of the ring-shaped zones ofthe diffractive structure at the maximum effective diameter hm.

In case that a short wavelength light source emitting a light having awavelength of not more than 550 nm, in particular, about 400 nm, asdescribed above, the refractive index change of the lens material, whichis caused by a slight wavelength change, becomes large. Therefore, whenthe slight wavelength change is caused, the axial chromatic aberrationis caused at the objective lens and the spherical aberration caused atthe objective lens is changed. For example, in case of an objective lenshaving a single lens structure, when the wavelength is shifted to 10 nmlonger wavelength side than the design wavelength, the sphericalaberration is changed in an over-corrected direction. In case of anobjective lens having a doublet structure, when the wavelength isshifted to 10 nm longer wavelength side than the design wavelength, thespherical aberration is changed in an over-corrected direction or in anunder-corrected direction, according to a power arrangement in the lensgroup.

The formula (20) is a condition to compensate the spherical aberrationchange caused at the objective lens, by a diffracting function of theaberration compensating optical element. If the optical path differencefunction has only second order coefficient of the optical pathdifference function (also referred to as “coefficient of a diffractivesurface), the condition is (P_(hf)/P_(hm))−2=0. However, in theaberration compensating optical element according to the presentinvention, higher order coefficients of the optical path differencefunction are used in order to precisely compensate the sphericalaberration change which is caused at the objective lens by a slightwavelength change from the design wavelength, with the diffractingfunction of the diffractive structure in the aberration compensatingoptical element. Therefore, it is preferable that the formula(P_(hf)/P_(hm))−2 has a value apart from 0 to a certain degree. If theformula (20) is satisfied, the above spherical aberration change isexcellently cancelled by the diffracting function.

In the above-described aberration compensating optical element, when awavelength of a light entering the diffractive structure is not morethan 550 nm, it is preferable that a diffraction efficiency of thediffractive structure becomes maximal. More preferably, the designwavelength of the objective lens is substantially the same as thewavelength at which the diffraction efficiency becomes maximal.

In the above-described aberration compensating optical element, it ispreferable that the aberration compensating optical element is a plasticlens. As an optical plastic material, for example, olefin resin,polymethylmethacrylate, styrene acrylonitrile, polycarbonate,thermosetting plastics, polystyrene or the like is exemplified.Preferable optical plastic material has an internal transmittance of notless than 80% when the transmitted light has the wavelength of not morethan 550 nm and the optical plastic has the thickness of 3 mm. In casethat the aberration compensating optical element is disposed in anoptical pickup device as an element separated from the objective lenslike the aberration compensating optical element according to thepresent invention, it is not strongly required that to thin theaberration compensating optical element or to downsize it like theobjective lens in order to obtain the operating distance and to lightenit. Therefore, a refraction-diffraction-integrated type of opticalelement lens which is made of the optical plastic material, can beeasily produced by the injection molding method or the like using a moldat a low cost.

In accordance with the fifth aspect of the present invention, an opticalsystem for carrying out at least one of a record of information on aninformation recording surface of an optical information recording mediumand a reproduction of information from the information recordingsurface; comprises:

a light source for emitting a light having a wavelength of not more than550 nm;

an objective lens made of a material having an Abbe constant of not morethan 95.0 at a d-line; and

any one of the above-described aberration compensating optical elements,which is disposed on an optical path between the light source and theobjective lens.

In accordance with the sixth aspect of the present invention, an opticalpickup device for carrying out at least one of a record of informationon an information recording surface of an optical information recordingmedium and a reproduction of information from the information recordingsurface; comprises:

an optical system comprising: a light source for emitting a light havinga wavelength of not more than 550 nm; an objective lens made of amaterial having an Abbe constant of not more than 95.0 at a d-line; andan aberration compensating optical element, which is disposed on anoptical path between the light source and the objective lens;

wherein the optical pickup device comprises the above-described opticalsystem as a condensing optical system.

According to the above-described optical system, it is possible torealize an optical system for optical pickup device and an opticalpickup device, which can compensate the axial chromatic aberration witha relatively simple structure even though a light source having a badmonochromaticity or a light source in which a wavelength of the lightsuddenly changes, is used in a high density storage optical pickupsystem or the like, and which can be manufactured in a low cost.

In the present invention, the optical information recoding mediumincludes not only a current optical information recoding medium having adisk-shape, for example, each type of CD, such as CD, CD-R, CD-RW,CD-Video, CD-ROM or the like, and each type of DVD, such as DVD,DVD-ROM, DVD-RAM, DVD-R, DVD-RW, DVD+RW or the like, and MD or the likebut also a next generation high density recording medium or the like.

The above-described optical pickup device according to the presentinvention, can be provided in a recorder and a reproducer for at leastone of a sound and an image, for example, a player or a drive which iscompatible with an optical information recording medium, such as CD,CD-R, CD-RW, CD-Video, CD-ROM, DVD, DVD-ROM, DVD-RAM, DVD-R, DVD-RW,DVD+RW, MD or the like, or an AV apparatus, a personal computer or otherinformation terminals into which the player or the drive isincorporated, or the like.

In accordance with the seventh aspect of the present invention, anaberration compensating optical element comprises:

a plastic lens having a single lens structure, and comprising adiffractive structure having a plurality of ring-shaped zone stepsformed on at least one surface of the plastic lens;

wherein the aberration compensating optical element is adapted for beingdisposed on an optical path between a light source and an objective lenshaving an image-side numerical aperture of not less than 0.75 andcomprising at least one plastic lens; and

wherein the aberration compensating optical element decreases a changeΔ3SA_(OBJ) in a third-order spherical aberration of the objective lens,which is caused by a refractive index change ΔN_(OBJ) of at least oneplastic lens contained in the objective lens due to a temperature changeof the objective lens, by using an inclination change of a marginal rayof an emergent light flux from the aberration compensating opticalelement, which is caused by a refractive index change ΔN_(AC) of theaberration compensating optical element due to a temperature change ofthe aberration compensating optical element.

As described above, the aberration compensating optical elementaccording to the present invention, comprises a plastic lens comprisinga diffractive structure having a plurality of ring-shaped zone stepsformed into substantially concentric circles on at least one surface ofthe plastic lens; wherein the aberration compensating optical element isadapted for being disposed on an optical path between a light source andan objective lens having an image-side numerical aperture of not lessthan 0.75 and comprising at least one plastic lens. A general opticalplastic material has a property that when the temperature rises, therefractive index thereof decreases and when the temperature falls, therefractive index thereof increases. An amount of the change in therefractive index of the optical plastic material, which is caused by thetemperature change, is larger than that of the optical glass material inthe number of zeroes. Therefore, the power of the whole system of theaberration compensating optical element which is a plastic lens, isconstant. The diffractive structure having a plurality of ring-shapedzone steps is formed into substantially concentric circles on at leastone surface of the aberration compensating optical element to suitablydistribute the power to the diffractive power as a diffractive lens andthe refractive power as a refractive lens. Thereby, it is possible toselect an amount of the inclination change of a marginal ray of anemergent light flux from the aberration compensating optical element,which is caused by changing the refractive index change of theaberration compensating optical element due to the temperature change.

As an optical plastic material, for example, olefin resin,polymethylmethacrylate, styrene acrylonitrile, polycarbonate,thermosetting plastics, polystyrene or the like is exemplified.Preferable optical plastic material has an internal transmittance of notless than 80% when the transmitted light has the wavelength of not morethan 550 nm and the optical plastic has the thickness of 3 mm.

When the above aberration compensating optical element is disposed on anoptical path between the light source and the objective lens comprisingat least one plastic lens, it is possible that the change in thethird-order spherical aberration of the objective lens, which is causedby a refractive index change of the plastic lens due to the temperaturechange, is changed so as to cancel it by the inclination change of themarginal ray of the emergent light flux from the aberration compensatingoptical element. Therefore, even though a high NA objective lens havingat least one plastic lens, which has a narrow usable temperature range,is used, the usable temperature range can be expanded by using theobjective lens with the aberration compensating optical elementaccording to the present invention. As a result, in a high densitystorage optical pickup system in which the objective lens having animage-side numerical aperture of not less than 0.75 is required, aplastic lens can be used as an objective lens. Therefore, it is possibleto decrease the cost of the optical pickup device.

In the above-described aberration compensating optical element, it ispreferable that the following formula (21) is satisfied:

P_(T1)<P_(T0)<P_(T2)  (21),

where P_(T0) is a paraxial power (mm⁻¹) of the aberration compensatingoptical element at a predetermined temperature T₀;

P_(T1) is a paraxial power (mm⁻¹) of the aberration compensating opticalelement at a temperature T₁ which is lower than the predeterminedtemperature T₀; and

P_(T2) is a paraxial power (mm⁻¹) of the aberration compensating opticalelement at a temperature T₂ which is higher than the predeterminedtemperature T₀.

Because the aberration compensating optical element satisfies the aboveformula (21), when the temperature rises, an inclination of an uppermarginal ray of the emergent light flux from the aberration compensatingoptical element is changed in a direction of decreasing it as comparedwith a previous temperature change, that is, in a clockwise direction onthe basis of the optical axis. This phenomenon has the same effect as aphenomenon that the magnification of the objective lens is changed in adirection of increasing it. Therefore, in the optical pickup opticaldevice, when the temperature rises, the change in the third-orderspherical aberration, which is caused by changing the temperature of theobjective lens, can be decreased so as to cancel it by using theaberration compensating optical element with the objective lens having atemperature property that the third-order spherical aberration ischanged in an under-corrected direction.

It is preferable that the predetermined temperature T₀ is 25° C. andthat the temperature difference between T₀ and T₁ and the temperaturedifference between T₀ and T₂ are 30° C.

It is preferable that the objective lens is one having a doublet lensstructure in which a first lens having a positive refractive power and asecond lens having a positive refractive power are arranged in an orderfrom a side of the objective lens; and at least the first lens is aplastic lens.

As an objective lens having a temperature property that the third-orderspherical aberration component thereof is changed in an under-correcteddirection when the temperature rises, the above-described objective lenshaving a doublet lens structure is exemplified. At least the first lensis a plastic lens. In order to decrease a cost and to lighten theoptical system, it is preferable that both of the first lens and thesecond lens are plastic lenses.

In case that the aberration compensating optical element is used withthe above-described objective lens having a doublet lens structure, itis preferable that the aberration compensating optical element accordingto the present invention satisfies the following formulas (22) and (23):

P_(R)<0  (22)

0<ΔP _(AC) /ΔT _(AC)<1×10⁻⁴  (23),

where P_(R) is a refractive power (mm⁻¹) of the aberration compensatingoptical element as a refractive lens; and

ΔP_(AC) is an amount of a change in a paraxial power (mm⁻¹) of theaberration compensating optical element, which is caused by thetemperature change ΔT_(AC) (° C.) of the aberration compensating opticalelement.

When the refractive power of the aberration compensating optical elementas a refractive lens satisfies the formula (22), a sign of an amount ofthe change (ΔP_(AC)/ΔT_(AC)) in the paraxial power of the aberrationcompensating optical element, which is caused by the temperature change,is positive. Therefore, when the temperature changes, the change in thethird-order spherical aberration of the objective lens having a doubletlens structure is decreased so as to cancel it. On the other hand, whenthe value of the formula (ΔP_(AC)/ΔT_(AC)) is smaller than the upperlimit of the formula (22), when the temperature changes, the third-orderspherical aberration of the objective lens having a doublet lensstructure is not too compensated. As described above, when the value ofthe formula (ΔP_(AC)/ΔT_(AC)) satisfies the formula (22), it is possibleto suitably compensate the spherical aberration change of the objectivelens having a doublet lens structure, which is caused by the temperaturechange.

It is preferable that the light source is one for emitting a lighthaving a wavelength of not more than 550 nm; and

wherein the following formula (24) is satisfied:

P_(λ1)<P_(λ0)<P_(λ2)  (24),

where P_(λ0) is a paraxial power (mm⁻¹) of the aberration compensatingoptical element at the wavelength λ₀ of the light emitted from the lightsource;

P_(λ1) is a paraxial power (mm⁻¹) of the aberration compensating opticalelement at a wavelength λ₁ which is 10 nm shorter than the wavelengthλ₀; and

P_(λ2) is a paraxial power (mm⁻¹) of the aberration compensating opticalelement at a wavelength λ₂ which is 10 nm longer than the wavelength λ₀.

As described above, because the aberration compensating optical elementhas a wavelength property to satisfy the formula (24), when a lighthaving a different wavelength from one having a predetermined wavelengthby a predetermined wavelength difference, enters the aberrationcompensating optical element, the axial chromatic aberration occurringat the aberration compensating optical element and the axial chromaticaberration occurring at the objective lens are cancelled to compensatethe axial chromatic aberration caused by the wavelength change.Therefore, the axial chromatic aberration is minimized at the condensingspot formed by condensing the light transmitting the aberrationcompensating optical element and the objective lens on the informationrecording surface of the optical information recording medium. By usingthe aberration compensating optical element according to the presentinvention with the objective lens, even though the axial chromaticaberration caused at the objective lens is not precisely compensated,the objective lens can be used as an objective lens for a high densityoptical information recording medium.

When the change in the third-order spherical aberration component of theobjective lens, which is caused by the temperature change, is decreasedso as to cancel it, in order to minimize the remaining aberration of thewhole system, it is preferable that the objective lens which is usedwith the aberration compensating optical element, satisfies the formula(25). Thereby, when the change in the third-order spherical aberrationcomponent of the objective lens, which is caused by the temperaturechange, is decreased so as to cancel it, it is possible to minimize theremaining aberration.

The following formulas (26) to (28) are conditions for decreasing thechange in the spherical aberration of the objective lens, which iscaused by the temperature change, so as to excellently cancel it and forminimizing the axial chromatic aberration at the condensing spot on theinformation recording surface of the optical information recordingmedium. In case that when the temperature changes, the change in thethird-order spherical aberration of the objective lens satisfies theformula (26) and the axial chromatic aberration occurring the objectivelens satisfies the formula (27), the diffractive structure having adiffractive power which satisfies the formula (28) is formed on theaberration compensating optical element. Thereby, the compensation forthe temperature property of the objective lens and the compensation forthe axial chromatic aberration occurring at the objective lens arecompatible.

|Δ3SA _(OBJ)|/|Δ5SA _(OBJ)|>1  (25),

−30.0×10⁻⁴<Δ3SA _(OBJ)/(ΔT _(OBJ) ·NA ⁴ ·f _(OBJ))<0  (26),

3×10⁻² <ΔfB _(OBJ) ·νd _(OBJ) /f _(OBJ)<14×10⁻²  (27),

1.0×10⁻² <P _(D)<10.0×10⁻²  (28),

where Δ3SA_(OBJ) is a change in a third-order spherical aberrationcomponent of a Zernike polynomial into which an aberration of theobjective lens is expanded, in case that a refractive index of theplastic lens in the objective lens is changed by ΔN_(OBJ) due to thetemperature change ΔT_(OBJ) (° C.) of the objective lens; the change inthe third-order spherical aberration being expressed by an RMS (rootmean square value) by a wavelength λ₀ of a light emitted from the lightsource; and a sign of the change in the third-order spherical aberrationbeing positive when the third-order spherical aberration component ischanged in an over-corrected direction, and being negative when thethird-order spherical aberration component is changed in anunder-corrected direction;

Δ5SA_(OBJ) is a change in a fifth-order spherical aberration componentof the Zernike polynomial into which the aberration of the objectivelens is expanded, in case that the refractive index of the plastic lensin the objective lens is changed by ΔN_(OBJ) due to the temperaturechange ΔT_(OBJ) (° C.) of the objective lens; the change in thefifth-order spherical aberration being expressed by an RMS (root meansquare value) by the wavelength λ₀ of a light emitted from the lightsource;

NA is a predetermined image-side numerical aperture which is requiredfor at least one of a record of information on an optical informationrecording medium and a reproduction of information from the opticalinformation recording medium;

f_(OBJ) is a focal length (mm) of the objective lens;

ΔfB_(OBJ) is an axial chromatic aberration (mm) occurring at theobjective lens when a light having a wavelength which is +10 nm longerthan the wavelength λ₀ of a light emitted from the light source entersthe objective lens;

νd_(OBJ) is a mean value of an Abbe constant of the first lens in theobjective lens at the d-line and an Abbe constant of the second lens atthe d-line; and

P_(D) is a paraxial power (mm⁻¹) of the diffractive structure and isdefined by the following equation:

P _(D)=Σ(−2·b _(2i) ·ni),

when an optical path difference function is defined by the followingequation:

Φbi=ni·(b _(2i) ·hi ² +b _(4i) ·hi ⁴ +b _(6i) ·hi ⁶+ . . . ),

as a function that an optical path difference Φbi added to a wavefronttransmitting through the diffractive structure formed on an i-th surfaceof the aberration compensating optical element, by the diffractivestructure formed on the i-th surface, is expressed by using a height hi(mm) from an optical axis; where b_(2i), b_(4i), b_(6i), . . . are asecond order coefficient of the optical path difference function, afourth order one, a sixth order one . . . , respectively, and ni is adiffraction order of a diffracted light having a maximum diffractedlight amount among a plurality of diffracted lights generated by thediffractive structure formed on the i-th surface.

It is preferable that the above-described aberration compensatingoptical element comprises one optical surface on which the diffractivestructure having a plurality of macroscopically plane ring-shaped zonesteps is formed, that is, one optical surface having a diffractivestructure having a plurality of ring-shaped zone steps formed on a planesurface and another optical surface opposite to the one optical surface,which has a concave refractive surface.

When the diffractive surface having the diffractive ring-shaped zonestructure is formed on a plane surface, the reflected light which isreflected by the diffractive structure, travels in a different directionfrom the incident light. Therefore, it can be prevented that a ghostsignal is detected by a detection system of the optical pickup device.Further, the diffractive structure can be precisely formed by theelectron beam drawing. The method for preparing the fine diffractivestructure having fine ring-shaped zones by using the electron beamdrawing is disclosed in “OPTICAL DESIGN, Journal of Optics Design Group,No. 20, 2000.2.25, pp.26-31”. The optical surface opposite to the planesurface on which the diffractive structure is formed, is a concaverefractive surface. When the refractive power of the concave surface hasan absolute value which is substantially the same as that of thediffractive power of the diffractive structure and the sign of therefractive power is opposite to that of the diffractive power, the powerof the whole system of the aberration compensating optical element canbe 0. Therefore, it becomes easy that the aberration compensatingoptical element is disposed in the parallel light flux.

In the above-described aberration compensating optical element, it ispreferable that the paraxial power P_(λ0) of the aberration compensatingoptical element is substantially zero at the wavelength λ₀ of the lightemitted from the light source. Therefore, it becomes easy that theaberration compensating optical element is disposed in the parallellight flux. In the concrete, wherein the following formulas (29) to (31)are satisfied:

P_(D)>0  (29)

P_(R)<0  (30)

−0.9<P _(D) /P _(R)<−1.1  (31).

It is preferable that the above-described aberration compensatingoptical element has a temperature property that when the temperaturerises, the paraxial power is changed in a direction of decreasing it,and the following formula (32) is satisfied:

P_(T2)<P_(T0)<P_(T1)  (32),

where P_(T0) is a paraxial power (mm⁻¹) of the aberration compensatingoptical element at a predetermined temperature T₀;

P_(T1) is a paraxial power (mm⁻¹) of the aberration compensating opticalelement at a temperature T₁ which is lower than the predeterminedtemperature T₀; and

P_(T2) is a paraxial power (mm⁻¹) of the aberration compensating opticalelement at a temperature T₂ which is higher than the predeterminedtemperature T₀.

Because the aberration compensating optical element satisfies the aboveformula (32), when the temperature rises, an inclination of an uppermarginal ray of the emergent light flux from the aberration compensatingoptical element is changed in a direction of increasing it as comparedwith a previous temperature change, that is, in a counterclockwisedirection on the basis of the optical axis. This phenomenon has the sameeffect as a phenomenon that the magnification of the objective lens ischanged in a direction of decreasing it. Therefore, in the opticalpickup optical device, when the temperature rises, the change in thethird-order spherical aberration, which is caused by changing thetemperature of the objective lens, can be decreased so as to cancel itby using the aberration compensating optical element with the objectivelens having a temperature property that the third-order sphericalaberration is changed in an over-corrected direction.

It is preferable that the predetermined temperature T₀ is 25° C. andthat the temperature difference between T₀ and T₁ and the temperaturedifference between T₀ and T₂ are 30° C.

As an objective lens having a temperature property that the third-orderspherical aberration component is changed in an over-corrected directionwhen the temperature rises, a plastic lens having a single lensstructure is exemplified. When the numerical aperture of the plasticlens having a single lens structure increases, the usable temperaturerange thereof becomes very narrow (the change in the sphericalaberration of the plastic lens having a single lens structure, which iscaused by the temperature change, is about 5 to 10 times larger thanthat of the plastic lens having a doublet lens structure, which has thesame focal length, image-side numerical aperture, working wavelength andthe magnification.) Therefore, a temperature controlling device forcontrolling the temperature of the plastic lens is specially required.As a result, the increase in the manufacturing cost of the opticalpickup device and the complication of the optical pickup device arecaused. By using the aberration compensating optical element accordingto the present invention with a high NA plastic lens having a singlelens structure, the usable temperature range of a high NA plastic lenshaving a single lens structure can be expanded with a simple structureand at a low cost.

It is preferable that at least one ring-shaped zone step having a stepdistance Δ (mm) in a direction of an optical axis between adjacent stepsof the plurality of ring-shaped zone steps is formed within an effectivediameter so that m, defined by following equations:

m=INT(Y),

Y=Δ×(n−1)/(λ₀×10⁻³)  (33),

is an integer except 0 and ±1,

where INT(Y) is an integer obtained by rounding Y, λ₀ is the wavelength(nm) of the light emitted from the light source, and n is a refractiveindex of the aberration compensating optical element at the wavelengthλ₀ (nm).

The above formula (33) has a meaning that the difference Δ (mm) in adirection of an optical axis between adjacent steps of the plurality ofring-shaped zone steps of the aberration compensating optical element isdetermined so that a diffracted light amount of the diffracted lighthaving a diffraction order which is two or more, is larger than those ofthe diffracted lights having the other diffraction orders, when theincident light flux is diffracted by the diffractive structure formed onthe optical surface of the aberration compensating optical element.Thereby, as compared with the case that the difference Δ is determinedso that the first order diffracted light has a maximum diffracted lightamount, the minimum value of the interval of the adjacent ring-shapedzone steps is m times larger. Therefore, a light amount loss occurringdue to the phase non-matching portion caused by a transfer defect of thering-shaped zone steps in the molding, is decreased. Because the numberof the ring-shaped zone steps is decreased by 1/m, it is possible toshorten the processing time of a die for the molding.

It is preferable that the above-described aberration compensatingoptical element comprises two diffractive structures having a pluralityof ring-shaped zone steps formed on both surfaces. By distributing thepower of the diffractive structure to two optical surface, a minimumvalue of the interval of the adjacent ring-shaped zone steps in theoptical axis direction, is larger as compared with the case that thediffractive structure is formed on only one surface. Therefore, a lightamount loss occurring due to the phase non-matching portion caused by atransfer defect of the ring-shaped zone steps in the molding, isdecreased.

In the above-described aberration compensating optical element, it ispreferable that the diffractive structure has such a sphericalaberration property that a spherical aberration of an emergent lightflux is changed in an under-corrected direction or an over-correcteddirection when a wavelength of an incident light flux is shifted to alonger wavelength side; and

when the diffractive structure is formed so as to satisfy the followinginequality:

0.2≦|(P _(hf) /P _(hm))−2|≦6.0  (34),

where P_(hf) is a first interval in a direction to perpendicular to anoptical axis of the diffractive structure between adjacent steps of thering-shaped zones of the diffractive structure at a diameter hf which isa half of a maximum effective diameter hm, and P_(hm) is a secondinterval in the direction to perpendicular to the optical axis of thediffractive structure between adjacent steps of the ring-shaped zones ofthe diffractive structure at the maximum effective diameter hm.

In case that a short wavelength light source emitting a light having awavelength of not more than 550 nm, in particular, about 400 nm, therefractive index change of the lens material, which is caused by aslight wavelength change, becomes large. Therefore, when the slightwavelength change is caused, the axial chromatic aberration is caused atthe objective lens and the spherical aberration caused at the objectivelens is changed. For example, in case of an objective lens having asingle lens structure, when the wavelength is shifted to 10 nm longerwavelength side than the design wavelength, the spherical aberration ischanged in an over-corrected direction. In case of an objective lenshaving a doublet structure, when the wavelength is shifted to 10 nmlonger wavelength side than the design wavelength, the sphericalaberration is changed in an over-corrected direction or in anunder-corrected direction, according to a power arrangement in the lensgroup.

The formula (34) is a condition to compensate the spherical aberrationchange caused at the objective lens, by a diffracting function of theaberration compensating optical element. If the optical path differencefunction has only second order coefficient of the optical pathdifference function (also referred to as “coefficient of a diffractivesurface), the condition is (P_(hf)/P_(hm))−2=0. However, in theaberration compensating optical element according to the presentinvention, higher order coefficients of the optical path differencefunction are used in order to precisely compensate the sphericalaberration change which is caused at the objective lens by a slightwavelength change from the design wavelength, with the diffractingfunction of the diffractive structure in the aberration compensatingoptical element. Therefore, it is preferable that the formula(P_(hf)/P_(hm))−2 has a value apart from 0 to a certain degree. If theformula (34) is satisfied, the above spherical aberration change isdecreased so as to excellently cancel it by the diffracting function.

In the above-described aberration compensating optical element, when awavelength of a light entering the diffractive structure is not morethan 550 nm, it is preferable that a diffraction efficiency of thediffractive structure becomes maximal. More preferably, the designwavelength of the objective lens is substantially the same as thewavelength at which the diffraction efficiency becomes maximal.

In accordance with the eighth aspect of the present invention, anoptical system for carrying out at least one of a record of informationon an information recording surface of an optical information recordingmedium and a reproduction of information from the information recordingsurface; comprises:

a light source;

an objective lens having an image-side numerical aperture of not lessthan 0.75 and comprising at least one plastic lens; and

any one of the above-described aberration compensating optical elements,which is disposed on an optical path between the light source and theobjective lens.

In accordance with the ninth aspect of the present invention, an opticalpickup device for carrying out at least one of a record of informationon an information recording surface of an optical information recordingmedium and a reproduction of information from the information recordingsurface; comprises:

an optical system comprising: a light source; an objective lens havingan image-side numerical aperture of not less than 0.75 and comprising atleast one plastic lens; an aberration compensating optical element,which is disposed on an optical path between the light source and theobjective lens;

wherein the optical pickup device comprises the above-described opticalsystem as a condensing optical system.

According to the above-described optical system and the optical pickupdevice, even though a high NA plastic objective lens is used in a highdensity storage optical pickup system or the like, the change in thespherical aberration occurring at the objective lens, which is caused bythe temperature change, can be minimized with a relatively simplestructure.

In accordance with the tenth aspect of the present invention, in arecorder and a reproducer, the above-described optical pickup device isprovided. It is possible to carry out the record of at least one of asound and an image and the reproduction of at least one of a sound andan image, respectively.

In this specification, the diffractive surface is a surface having afunction of diffracting the incident light flux by providing a relief ona surface of the optical element, for example, a surface of a lens. Incase of one surface having a region that the diffraction occurs and aregion that the diffraction does not occur, the diffractive surface is aregion that the diffraction occurs. The diffractive structure or thediffraction pattern is the region that the diffraction occurs. As ashape of the relief, for example, a plurality of ring-shaped zones whichare formed into substantially concentric circles around the optical axison the surface of the optical element, are known. Further, the pluralityof ring-shaped zones have a sectional form in which each ring-shapedzone step has a shape of saw teeth or a shape of steps on the crosssection including the optical axis. The shape of the relief includes theabove-described shape.

In this specification, the record of information and the reproduction ofinformation are that the information is recorded on the informationrecording surface of the optical information recording medium and thatthe information recorded on the information recording surface isreproduced, respectively. The condensing optical system according to thepresent invention, may be used to carry out the record only or thereproduction only, or to carry out both the record and the reproduction.The condensing optical system may carry out the record for one opticalinformation recording medium and carry out the reproduction for anotheroptical information recording medium. Further, the condensing opticalsystem may carry out the record or the reproduction for one opticalinformation recording medium and carry out both the record and thereproduction for another optical information recording medium. In thespecification, the reproduction includes that the information is onlyread out.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not intendedas a definition of the limits of the present invention, and wherein;

FIG. 1 is a view for schematically showing on optical pickup deviceaccording to the first embodiment of the present invention;

FIG. 2A is a side view showing an aberration compensating opticalelement shown in FIG. 1, and FIG. 2B is a plan view showing anaberration compensating optical element shown in FIG. 1, which is viewedin the direction of the arrow A;

FIG. 3 is a view showing an example of an objective lens which causesthe axial chromatic aberration to be compensated by the aberrationcompensating optical element according to Examples 1 to 3 of the presentinvention, and is an optical path diagram of the objective lens;

FIG. 4 is a graph showing the spherical aberration and the axialchromatic aberration of the objective lens shown in FIG. 3 at thewavelength of 405±10 nm;

FIG. 5 is an optical path diagram of the optical system for the opticalpickup device according to Example 1;

FIG. 6 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 1 at the wavelength of 405±10 nm;

FIG. 7 is an optical path diagram of the optical system for the opticalpickup device according to Example 2;

FIG. 8 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 2 at the wavelength of 405±10 nm;

FIG. 9 is an optical path diagram of the optical system for the opticalpickup device according to Example 3;

FIG. 10 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 3 at the wavelength of 405±10 nm;

FIG. 11 is a schematic view for showing on optical pickup deviceaccording to the second embodiment of the present invention;

FIG. 12A is a side view showing an aberration compensating opticalelement shown in FIG. 11, and FIG. 12B is a plan view showing anaberration compensating optical element shown in FIG. 11, which isviewed in the direction of the arrow A;

FIG. 13 is a view showing an example of an objective lens in which theaxial chromatic aberration is compensated by the aberration compensatingoptical element according to Examples 4 to 6 of the present invention,and is an optical path diagram of the objective lens;

FIG. 14 is a graph showing the spherical aberration and the axialchromatic aberration of the objective lens shown in FIG. 13 at thewavelength of 405±10 nm;

FIG. 15 is an optical path diagram of the optical system for the opticalpickup device according to Example 4;

FIG. 16 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 4 at the wavelength of 405±10 nm;

FIG. 17 is an optical path diagram of the optical system for the opticalpickup device according to Example 5;

FIG. 18 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 5 at the wavelength of 405±10 nm;

FIG. 19 is an optical path diagram of the optical system for the opticalpickup device according to Example 6;

FIG. 20 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 6 at the wavelength of 405±10 nm;

FIG. 21 is an optical path diagram of the optical system according toExample 7;

FIG. 22 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system according to Example 7 at thewavelength of 405±1 nm;

FIG. 23 is an optical path diagram of the optical system according toExample 8;

FIG. 24 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system according to Example 8 at thewavelength of 405±1 nm;

FIG. 25 is a view showing the result that the axial chromatic aberrationcaused at the objective lens shown in Table 4 due to the instantwavelength change of the light emitted from the light source iscompensated by the aberration compensating optical elements according toExamples 7 and 8;

FIG. 26 is a view showing the result that the third-order sphericalaberration changed at the objective lens shown in Table 4 is compensatedby the aberration compensating optical elements according to Examples 7and 8;

FIG. 27 is a view showing the spherical aberration change of the plasticobjective lens having a doublet lens structure according to an earliertechnology of the inventor, as a function of the temperature change,wherein the plastic objective lens has an NA of 0.85, a design referencewavelength of 405 nm, a focal length of 1.76 and a design referencetemperature of 25° C.; and

FIG. 28 is a graph showing the spherical aberration and the axialchromatic aberration of the objective lens according to Table 4 at thewavelength of 405±1 nm.

PREFERRED EMBODIMENT OF THE INVENTION

First Embodiment:

Hereinafter, the first embodiment of the present invention will beexplained with reference to the drawings. FIG. 1 is a view forschematically showing on optical pickup device including a condensingoptical system for optical pickup device according to the firstembodiment of the present invention. FIG. 2A is a side view showing anaberration compensating optical element shown in FIG. 1, and FIG. 2B isa plan view which is viewed in the direction of the arrow A.

The optical pickup device 1 shown in FIG. 1 comprises a semiconductorlaser 2 as a light source, a hybrid refractive-diffractive aberrationcompensating optical element 3 and an objective lens 4. Thesemiconductor laser 2 is a GaN blue-violet laser for emitting the lightflux having a wavelength of about 400 nm. As a light source for emittingthe light flux having a wavelength of about 400 nm, not only the aboveGaN blue-violet laser but also an SHG blue-violet laser may be used.

As shown in FIGS. 2A and 2B, a diffraction pattern p having the form ofsubstantially concentric circles is provided on a surface S1 which is onthe semiconductor laser 2 side of the aberration compensating opticalelement 3, and which is a plane optical surface. The power of thediffraction pattern p is determined so as to satisfy the formula (15).The intervals of the diffraction pattern p, in the directionperpendicular to the optical axis thereof, are determined by the formula(12). Further, a surface S2 which is on the objective lens 4 side of theaberration compensating optical element 3, is a concave surface having anegative refractive power. By setting the absolute value of the power ofthe diffraction pattern and that of the power of the concave surface soas to be equal to each other, the whole power of aberration compensatingoptical element 3 becomes 0.

The diffraction pattern having the form of substantially concentriccircles may be provided on a surface which is on the objective lens 4side of the aberration compensating optical element 3 or on the bothsurfaces which are on the light source 2 side and the objective lens 4side. In this embodiment, the whole power of the aberration compensatingoptical element 3 is 0. However, the whole power of the aberrationcompensating optical element 3 may be positive or negative. Further, inthis embodiment, the diffraction pattern of the aberration compensatingoptical element 3 is formed in substantially concentric circles aroundthe optical axis thereof. However, other diffraction patterns may beprovided.

In the optical pickup device 1 of FIG. 1, the aberration compensatingoptical element 3 is constructed by using one lens. However, theaberration compensating optical element 3 may be constructed by using aplurality of lenses as long as the structure thereof does not departfrom the scope of the present invention.

Further, in the optical pickup device 1 of FIG. 1, the aberrationcompensating optical element 3 is disposed as an element which isseparated from the objective lens 4. However, the aberrationcompensating optical element 3 may be united with the objective lens 4by using a lens frame, an adhesive or the like. In this case, becausethe tracking thereof is unitedly controlled with a two-axis actuator 10by moving the aberration compensating optical element 3 and theobjective lens 4 together, an excellent tracking property can beobtained.

The objective lens 4 of FIG. 1 is one for condensing the emergent lightflux from the aberration compensating optical element 3, on aninformation recording surface 5 a of the optical disk 5 within thediffraction limited. The objective lens 4 is composed of one lens andhas at least one a spherical surface. The objective lens 4 comprises aflange portion 4 a having a surface extending perpendicularly to theoptical axis. The objective lens 4 can be accurately attached to theoptical pickup device 1 by the flange portion 4 a. It is preferable thatthe numerical aperture on the optical disk 5 side of the objective lens4 is not less than 0.65. More preferably, the numerical aperture is notless than 0.75. In this embodiment, the objective lens 4 is composed ofone lens. However, the objective lens may be composed of two or morelenses.

A divergent light flux emitted from the semiconductor laser 2 transmitsthrough a polarization beam splitter 6. The light flux passes through acollimate lens 7 and a quarter-wave plate 8 to become a circularlypolarized parallel light flux. After the light flux transmits throughthe aberration compensating optical element 3 and a stop 9, the lightflux becomes a spot formed by the objective lens 4 on the informationrecording surface 5 a through a transparent substrate 5 b of the highdensity storage optical desk 5. The objective lens 4 is focused andtracked by a two-axis actuator 10 disposed there around.

The reflected light flux modulated by a information pit on theinformation recording surface 5 a, transmits through the objective lens4, the aberration compensating optical element 3, the quarter-wave plate8 and the collimate lens 7. Then, the light flux becomes a convergedlight flux, and is reflected by the polarization beam splitter 6. Bytransmitting the light flux through the cylindrical lens 11, theastigmatism is given and the light flux is converged at a photo-detector12. Then, the information recorded on the optical disk 5 can be read byusing an output signal of the photo-detector 12.

In this embodiment, because the diffraction pattern having the form ofsubstantially concentric circles is provided on the optical surface ofthe aberration compensating optical element 3 as described above, theaxial chromatic aberration having an opposite sign to the axialchromatic aberration caused at the objective lens 4 and having the sameabsolute value as the axial chromatic aberration caused at the objectivelens 4, is generated by the aberration compensating optical element 3for the oscillation wavelength of the semiconductor laser 2. Therefore,the light flux emitted from the semiconductor laser 2 can be condensedon the information recording surface 5 a of the optical disk 5 withoutcausing the axial chromatic aberration by transmitting the light fluxthrough the aberration compensating optical element 3 and the objectivelens 4.

The optical pickup device 1 shown in FIG. 1, can be provided in arecorder and a reproducer for at least one of a sound and an image, forexample, a player or a drive which is compatible with an opticalinformation recording medium, such as CD, CD-R, CD-RW, CD-Video, CD-ROM,DVD, DVD-ROM, DVD-RAM, DVD-R, DVD-RW, DVD+RW, MD or the like, or an AVapparatus, a personal computer or other information terminals into whichthe player or the drive is incorporated, or the like.

EXAMPLES

In FIGS. 3 and 4, an example of the objective lens which causes theaxial chromatic aberration to be compensated by the aberrationcompensating optical element according to the present invention, isshown. FIG. 3 is an optical path diagram of the objective lens (focallength 1.76 mm, image-side numerical aperture 0.85) which causes theaxial chromatic aberration to be compensated by the aberrationcompensating optical element according to the present invention. Theobjective lens is made of olefin resin having the Abbe constant of 56.5at the d-line. Further, FIG. 4 is a graph showing the sphericalaberration and the axial chromatic aberration of the objective lens atthe wavelength of 405±10 nm. FIG. 4 shows that when the wavelength isshifted to the long wavelength side by 10 nm, the focal point is shiftedto the over side about 3 μm.

Next, Examples 1 to 3 of the optical system for an optical pickup deviceaccording to the present invention, will be explained. When the opticalaxis direction is “X-axis”, the height in the direction perpendicular tothe optical axis is “h”, and the radius of curvature of the refractivesurface is “r”, they are represented by the following formula (35)$\begin{matrix}{X = {\frac{h^{2}/r}{1 + \sqrt{1 - {\left( {1 + \kappa} \right){h^{2}/r^{2}}}}} + {\sum\limits_{i = 2}\quad {A_{2i}h^{2i}}}}} & (35)\end{matrix}$

where κ is the coefficient of circular cone and A_(2i) is thecoefficient of aspheric surface.

The diffractive surface having ring-shaped zones provided on the lensaccording to the present examples, can be represented by the followingformula (36) when the optical path difference function is Φb.$\begin{matrix}{\Phi_{b} = {n{\sum\limits_{j = 1}\quad {b_{2j}h^{2j}}}}} & (36)\end{matrix}$

where n is the diffraction order of the diffracted light having themaximum diffracted light amount among the diffracted lights generated bythe diffractive surface, h is the height perpendicular to the opticalaxis, and b_(2j) is the coefficient of the optical path differencefunction.

Example 1

In Table 1, the data relating to the optical system for the opticalpickup device according to Example 1, are shown. In the FIG. 5, theoptical path diagram of the optical system for the optical pickup deviceaccording to Example 1, is shown. In Table 1, r is the radius ofcurvature, d is the surface interval, N_(λ) is the refractive index atthe wavelength of 405 nm, and ν_(d) is the Abbe constant.

TABLE 1 Surface No. r (mm) d (mm) N_(λ) ν_(d) Note 0 ∞ Light source 1−27.620 1.500 1.52491 56.5 Aberration 2 28.168 5.000 compensatingoptical element 3 2.074 2.400 1.52491 56.5 Objective 4 8.053 0.100 lens5 0.863 1.100 1.52491 56.5 6 ∞ 0.240 7 ∞ 0.100 1.61949 30.0 Transparent8 ∞ substrate Coefficients of aspherical surface First Second ThirdFourth Fifth surface surface surface surface surface κ −5.4091E+001.2966E+00 −1.2955E−01 4.7554E+01 −7.1425E−01 A₄ −1.1312E−05 −4.2412E−07−3.7832E−03 1.3641E−02 1.3647E−01 A₆ 5.1667E−04 −2.9201E−02 −5.3414E−02A₈ −1.1780E−03 −9.3339E−03 3.0269E−01 A₁₀ −2.0628E−04 3.3011E−02−1.6898E−01 A₁₂ 2.5941E−05 −2.2626E−02 A₁₄ 1.4917E−04 A₁₆ −5.1578E−05Coefficients of diffractive surface First Second surface surface b₂−4.7281E−03 −4.6816E−03

In Example 1, by making the aberration compensating optical element ofolefin resin and forming the diffractive surfaces on the both opticalsurfaces which are on the light source side and the objective lens side,the axial chromatic aberration caused at the objective lens wascompensated. Further, the total power of each optical surface was 0 byforming each optical surface so as to have the negative refractive powerand the diffractive power having the same absolute value as therefractive power. Therefore, the emergent light flux diameter was notchanged in comparison with the incident light flux diameter. Further,the coefficient of the optical path difference function shown in Table1, was determined so that the second-order diffracted light had amaximum diffracted light amount.

FIG. 6 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 1 at the wavelength of 405±10 nm. FIG. 6 shows thatthe focal point is hardly shifted regardless of the wavelength.

The diffractive power of the aberration compensating optical element wasdetermined so that the axial chromatic aberration caused in the compoundsystem including the objective lens was over-corrected. Further, theaberration remained in the lens so that the spherical aberration causedin the compound system was over-corrected on the long wavelength sideand was under-corrected on the short wavelength side. The shift of thebest image point, which was caused by changing the wavelength, wasminimized by crossing the graph of the spherical aberration of thereference wavelength and ones of the spherical aberrations on the longwavelength side and the short wavelength side. Therefore, the defocuscomponent of the wavefront aberration, which was caused by the modehopping of +1 nm, could be decreased to 0.002λrms (calculated value).

In case that the chromatic aberration caused in the compound system iscompensated as described above, the interval of the diffractingring-shaped zone on the aberration compensating optical element can beexpanded in comparison with the case that the shift of the best imagepoint, which is caused by changing the wavelength, is minimized byalmost perfectly compensating the axial chromatic aberration of thecompound system and by almost perfectly compensating the sphericalaberration on the long wavelength side and the spherical aberration onthe short wavelength side. Therefore, the light amount loss caused bythe form error of the diffracting ring-shaped zone structure in themanufacture thereof can be minimized.

Example 2

In Table 2, the data relating to the optical system for the opticalpickup device according to Example 2, are shown. In FIG. 7, the opticalpath diagram of the optical system for the optical pickup deviceaccording to Example 2, is shown. In Table 2, r is the radius ofcurvature, d is the surface interval, N_(λ) is the refractive index atthe wavelength of 405 nm, and ν_(d) is the Abbe constant.

TABLE 2 Surface No. r (mm) d (mm) N_(λ) ν_(d) Note 0 ∞ Light source 1 ∞1.000 1.52491 56.5 Aberration 2 14.165 5.000 compensating opticalelement 3 2.074 2.400 1.52491 56.5 Objective lens 4 8.053 0.100 5 0.8631.100 1.52491 56.5 6 ∞ 0.240 7 ∞ 0.100 1.61949 30.0 Transparent 8 ∞substrate Coefficients of aspherical surface Second Third Fourth Fifthsurface surface surface surface κ −1.0003E+00 −1.2955E−01 4.7554E+01−7.1425E−01 A₄ −1.0589E−04 −3.7832E−03 1.3641E−02 1.3647E−01 A₆5.1667E−04 −2.9201E−02 −5.3414E−02 A₈ −1.1780E−03 −9.3339E−03 3.0269E−01A₁₀ −2.0628E−04 3.3011E−02 −1.6898E−01 A₁₂ 2.5941E−05 −2.2626E−02 A₁₄1.4917E−04 A₁₆ −5.1578E−05 Coefficients of diffractive surface Firstsurface b₂ −9.0435E−03 b₄ 3.1856E−05

In Example 2, by making the aberration compensating optical element ofolefin resin and forming a diffractive surface which is amacroscopically plane surface, on the optical surface of the lightsource side, that is, preparing the diffractive structure on the opticalsurface of the light source side so that the diffractive structurecomprises a plurality of ring-shaped zone steps having the form of thesubstantially concentric circles which are formed on a plane surface,the axial chromatic aberration caused at the objective lens wascompensated. Further, the power of the whole system in the aberrationcompensating optical element was 0 by forming the optical surface of theobjective lens side so as to have the negative refractive power and byforming the optical surface of the light source side so as to have thediffractive power having the same absolute value as the refractivepower. Further, the coefficient of the optical path difference functionshown in Table 2, was determined so that the second-order diffractedlight had a maximum diffracted light amount.

FIG. 8 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 2 at the wavelength of 405±10 nm. FIG. 8 shows thatthe focal point is hardly shifted regardless of the wavelength.

Example 3

In Table 3, the data relating to the optical system for the opticalpickup device according to Example 3, are shown. In FIG. 9, the opticalpath diagram of the optical system for the optical pickup deviceaccording to Example 3, is shown. In Table 3, r is the radius ofcurvature, d is the surface interval, N_(λ) is the refractive index atthe wavelength of 405 nm, and ν_(d) is the Abbe constant.

TABLE 3 Surface No. r (mm) d (mm) N_(λ) ν_(d) Note 0 ∞ Light source 1−17.495 1.000 1.52491 56.5 Aberration 2 ∞ 5.000 compensating opticalelement 3 2.074 2.400 1.52491 56.5 Objective lens 4 8.053 0.100 5 0.8631.100 1.52491 56.5 6 ∞ 0.240 7 ∞ 0.100 1.61949 30.0 Transparent 8 ∞substrate Coefficients of aspherical surface First Third Fourth Fifthsurface surface surface surface κ 2.7605E+01 −1.2955E−01 4.7554E+01−7.1425E−01 A₄ 1.9423E−03 −3.7832E−03 1.3641E−02 1.3647E−01 A₆−1.4942E−03 5.1667E−04 −2.9201E−02 −5.3414E−02 A₈ −1.1780E−03−9.3339E−03 3.0269E−01 A₁₀ −2.0628E−04 3.3011E−02 −1.6898E−01 A₁₂2.5941E−05 −2.2626E−02 A₁₄ 1.4917E−04 A₁₆ −5.1578E−05 Coefficients ofdiffractive surface First surface b₂ −7.5000E−03 b₄ 3.3173E−04 b₆−4.0384E−04 b₈ 3.8224E−06 b₁₀ −1.0509E−06

In Example 3, by making the aberration compensating optical element ofolefin resin and forming the diffractive surfaces on the opticalsurfaces of the light source side, the axial chromatic aberration causedat the objective lens was compensated. Further, the power of the wholesystem in the aberration compensating optical element was 0 by formingthe optical surface of the light source side so as to have the negativerefractive power and the diffractive power having the same absolutevalue as the refractive power, and by forming the optical surface of thelight source side so as to be a plane surface. Further, the coefficientof the optical path difference function shown in Table 3, was determinedso that the second-order diffracted light had a maximum diffracted lightamount. In this example, the terms of the optical path differencefunction, to which the second or higher order coefficients were given,were used. The diffractive surface had a spherical aberration propertyto shift the spherical aberration in an under-corrected direction whenthe wavelength was shifted to the long wavelength side. Then, the changein the spherical aberration, which was caused at the objective lens whenthe wavelength was shifted to the long wavelength side, was cancelled bythe spherical aberration of the diffractive surface.

FIG. 10 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 3 at the wavelength of 405±10 nm. FIG. 10 showsthat the focal point is hardly shifted regardless of the wavelength.

Second Embodiment:

The inventor and the like proposed an objective lens having a doubletlens structure as a plastic objective lens which is suitable for asystem for a high density optical information recording medium, such asa high density DVD system, as disclosed in Japanese Patent ApplicationNo. Tokugan 2001-256422.

However, in a high NA plastic objective lens having the NA of about0.85, when the temperature changes, the change in the sphericalaberration, which is caused by the refractive index change of plastic,is large because it increases in proportion to the fourth power of NA.Therefore, in practical use, the above change becomes a significantproblem. As an example of the spherical aberration change, FIG. 27 showsthe spherical aberration change of the plastic objective lens having adoublet lens structure, as a function of the temperature change, whereinthe plastic objective lens has an NA of 0.85, a design referencewavelength of 405 nm, a focal length of 1.76 and a design referencetemperature of 25° C. In Table 4, r is the radius of curvature, d is thesurface interval, and N₄₀₄, N₄₀₅ and N₄₀₆ are the refractive indexes atthe wavelength of 404 nm, 405 nm and 406 nm, respectively.

TABLE 4 Surface No. r (mm) d (mm) N₄₀₄ N₄₀₅ N₄₀₆ Note 0 ∞ Light source 11.959 2.450 1.52485 1.52469 1.52454 Objective 2 10.207 0.050 lens 30.892 1.100 1.52485 1.52469 1.52454 4 ∞ 0.201 5 ∞ 0.100 1.61990 1.619501.61909 Transparent 6 ∞ substrate Coefficients of aspherical surfaceFirst Second Third surface surface surface κ −1.6847E−01 8.3955E+01−7.4973E−01 A₄ −4.8527E−03 5.4087E−03 1.2350E−01 A₆ −3.6816E−04−6.1028E−03 1.6589E−02 A₈ −3.6905E−04 −1.4958E−02 1.8042E−01 A₁₀−2.1477E−04 1.9993E−02 −1.0532E−01 A₁₂ −6.5049E−05 −1.7848E−02 A₁₄1.3300E−04 A₁₆ −4.2576E−05

In order to record information on an optical disk or reproduceinformation from an optical disk, it is required that the performance ofthe whole optical system in a pickup device satisfies Marechal'slimitation that the wavefront aberration is not more than 0.07λrms whereλ is the wavelength. Because not only an objective lens but also otheroptical elements, for example, a collimator, a prism and the like areincluded in a practical optical system for an optical pickup device, theupper limit of the wavefront aberration, which is permissible for theobjective lens, becomes about 0.03λrms. The operation guaranteetemperature of an optical disk player is about 85° C. However, at thetemperature of 85° C., the wavefront aberration of the objective lensexceeds 0.03λrms as shown in FIG. 27. Therefore, there is somepossibility that the record of information on an optical disk or thereproduction of information from an optical disk cannot be stablycarried out.

From formula (1), the upper limit of the axial chromatic aberration,which is permissible for the objective lens used in a high densityoptical pickup system, is calculated. From formula (1), when thewavelength of a light to be used is 405 nm, in order for the wavelengthaberration to satisfy not more than 0.03λrms, a maximum amount ofdefocus (axial chromatic aberration) ΔfB_(MAX) is

ΔfB_(MAX)=0.117 (μm)  (37).

FIG. 28 is a graph showing the spherical aberration and the axialchromatic aberration of the objective lens according to Table 4 at thewavelength of 405±1 nm. In case of a blue-violet semiconductor laserlight source, it is discussed that the wavelength is shifted about 1 nmby the mode hopping. As shown in FIG. 28, in this objective lens, whenthe wavelength is shifted from 405 nm to 406 nm, the axial chromaticaberration of about 0.25 μm is caused. As a result, the axial chromaticaberration exceeds 0.117 μm which is the upper limit of the axialchromatic aberration, which is permissible for the objective lens.

Hereinafter, the second embodiment of the present invention will beexplained with reference to the drawings.

FIG. 11 is a view for schematically showing on optical pickup deviceincluding an optical system for optical pickup device according to thesecond embodiment of the present invention. FIG. 12A is a side viewshowing an aberration compensating optical element shown in FIG. 11, andFIG. 12B is a plan view which is viewed in the direction of the arrow A.

The optical pickup device 101 shown in FIG. 11 comprises a semiconductorlaser 102 as a light source, a hybrid refractive-diffractive aberrationcompensating optical element 114 and an objective lens 113. Thesemiconductor laser 102 is a GaN blue-violet laser for emitting thelight flux having a wavelength of about 400 nm. As a light source foremitting the light flux having a wavelength of about 400 nm, not onlythe above GaN blue-violet laser but also an SHG blue-violet laser may beused.

As shown in FIGS. 12A and 12B, a diffraction pattern p having the formof substantially concentric circles is provided on a surface S11 whichis on the semiconductor laser 102 side of the aberration compensatingoptical element 114, and which is a plane optical surface. Further, asurface S12 which is on the objective lens 113 side of the aberrationcompensating optical element 114, is a concave surface having a negativerefractive power. By setting the absolute value of the power of thediffraction pattern and that of the power of the concave surface so asto be equal to each other, the whole power of aberration compensatingoptical element 114 becomes 0.

The diffraction pattern having the form of substantially concentriccircles may be provided on a surface which is on the objective lens 113side of the aberration compensating optical element 114 or on the bothsurfaces which are on the light source 102 side and the objective lens113 side. In this embodiment, the whole power of the aberrationcompensating optical element 114 is 0. However, the whole power of theaberration compensating optical element 114 may be positive or negative.Further, in this embodiment, the diffraction pattern of the aberrationcompensating optical element 114 is formed in substantially concentriccircles around the optical axis thereof. However, other diffractionpatterns may be provided.

In the optical pickup device 101 of FIG. 11, the aberration compensatingoptical element 104 is disposed as an element which is separated fromthe objective lens 103. However, the aberration compensating opticalelement 104 may be united with the objective lens 103 by using a lensframe, an adhesive or the like. In this case, because the trackingthereof is unitedly controlled with a two-axis actuator 110 by movingthe aberration compensating optical element 104 and the objective lens103 together, an excellent tracking property can be obtained.

The objective lens 113 is one for condensing the emergent light fluxfrom the aberration compensating optical element 114, on an informationrecording surface 105 a of the optical disk 105 within the diffractionlimited. The objective lens 113 is composed of two lenses which areassembled by a holding member 113 a and has at least one asphericalsurface. The numerical aperture on the optical disk 105 side is 0.85.The objective lens 113 comprises a flange portion 113 b having a surfaceextending perpendicularly to the optical axis. The objective lens 113can be accurately attached to the optical pickup device 101 by theflange portion 113 b.

A divergent light flux emitted from the semiconductor laser 102transmits through a polarization beam splitter 106. The light fluxpasses through a collimate lens 107 and a quarter-wave plate 108 tobecome a circularly polarized parallel light flux. After the light fluxtransmits through the aberration compensating optical element 114, thelight flux passes through a stop 109 and becomes a spot formed by theobjective lens 113 on the information recording surface 105 a through atransparent substrate 105 b of the high density storage optical desk105. The objective lens 113 is focused and tracked by an actuator 110disposed therearound.

The reflected light flux modulated by a information pit on theinformation recording surface 105 a, transmits through the objectivelens 113, the aberration compensating optical element 114, thequarter-wave plate 108 and the collimate lens 107. Then, the light fluxbecomes a converged light flux, and is reflected by the polarizationbeam splitter 106. By transmitting the light flux through thecylindrical lens 111, the astigmatism is given and the light flux isconverged at a photo-detector 112. Then, the information recorded on theoptical disk 105 can be read by using an output signal of thephoto-detector 112.

In this embodiment, because the aberration compensating optical element114 is a plastic lens and the diffraction pattern having the form ofsubstantially concentric circles is provided on the optical surface ofthe aberration compensating optical element 114 as described above, theaxial chromatic aberration having an opposite sign to the axialchromatic aberration caused at the objective lens 113 and having thesame absolute value as the axial chromatic aberration caused at theobjective lens 113, is generated by the aberration compensating opticalelement 114 for the oscillation wavelength of the semiconductor laser102. Therefore, the light flux emitted from the semiconductor laser 102is condensed on the information recording surface 105 a of the opticaldisk 105 without causing the axial chromatic aberration by transmittingthe light flux through the aberration compensating optical element 114and the objective lens 113.

In this embodiment, when the temperature of the objective lens 113 andthat of the aberration compensating optical element 114 rise byradiating heat from a focusing coil and a tracking coil which areattached near the objective lens 113 or by raising the externaltemperature for the optical pickup device 101, the refractive index ofthe plastic lens is changed so as to decrease it. Therefore, thethird-order spherical aberration component is changed in anunder-corrected direction at the objective lens 113. Then, because therefractive power of the whole system of the aberration compensatingoptical element 114 as a refractive lens satisfies the above formula(22), the emergent light flux from the aberration compensating opticalelement 114 becomes a converged light flux. The third-order sphericalaberration which was changed in an under-corrected direction iscancelled by entering the converged light flux into the objective lens113. As a result, even though the temperature rises, the light fluxemitted from the semiconductor laser 102 is condensed on the informationrecording surface 105 a in a state of minimizing the sphericalaberration by transmitting the light flux through the aberrationcompensating optical element 114 and the objective lens 113.

The optical pickup device shown in FIG. 11, can be provided in arecorder and a reproducer for at least one of a sound and an image, forexample, a player or a drive which is compatible with an opticalinformation recording medium, such as a next generation high densityrecording medium like a high density DVD or the like, CD, CD-R, CD-RW,CD-Video, CD-ROM, DVD, DVD-ROM, DVD-RAM, DVD-R, DVD-RW, DVD+RW, MD orthe like, or an AV apparatus, a personal computer or other informationterminals into which the player or the drive is incorporated, or thelike.

EXAMPLES

In FIGS. 13 and 14, an example of the objective lens which causes thechange in the spherical aberration due to the temperature change, andthe axial chromatic aberration to be compensated by the aberrationcompensating optical element according to Examples 4 to 6 in the secondembodiment of the present invention, is shown. FIG. 13 is an opticalpath diagram of the objective lens (focal length 1.76 mm, image-sidenumerical aperture 0.85) which causes the axial chromatic aberration tobe compensated by the aberration compensating optical element accordingto the present invention. The objective lens is made of olefin resinhaving the Abbe constant of 56.5 at the d-line. Further, FIG. 14 is agraph showing the spherical aberration and the axial chromaticaberration of the objective lens at the wavelength of 405±10 nm. FIG. 14shows that when the wavelength is shifted to the long wavelength side by10 nm, the focal point is shifted to the over side about 3 μm.

Next, Examples 4 to 8 of the optical system for an optical pickup deviceaccording to the second embodiment of the present invention, will beexplained. When the optical axis direction is “X-axis”, the height inthe direction perpendicular to the optical axis is “h”, and the radiusof curvature of the refractive surface is “r”, they are represented bythe following formula (38). $\begin{matrix}{X = {\frac{h^{2}/r}{1 + \sqrt{1 - {\left( {1 + \kappa} \right){h^{2}/r^{2}}}}} + {\sum\limits_{i = 2}\quad {A_{2i}h^{2i}}}}} & (38)\end{matrix}$

where κ is the coefficient of circular cone and A_(2i) is thecoefficient of aspheric surface.

The diffractive surface having ring-shaped zones provided on the lensaccording to the present examples, can be represented by the followingformula (39) when the optical path difference function is Φb.$\begin{matrix}{\Phi_{b} = {n{\sum\limits_{j = 1}\quad {b_{2j}h^{2j}}}}} & (39)\end{matrix}$

where n is the diffraction order of the diffracted light having themaximum diffracted light amount among the diffracted lights generated bythe diffractive surface, h is the height perpendicular to the opticalaxis, and b_(2j) is the coefficient of the optical path differencefunction.

Example 4

In Table 5, the data relating to the optical system for the opticalpickup device according to Example 4, are shown. In FIG. 15, the opticalpath diagram of the optical system for the optical pickup deviceaccording to Example 4, is shown.

TABLE 5 Surface No. r (mm) d (mm) N_(λ) ν_(d) Note 0 ∞ Light source 1−27.620 1.500 1.52491 56.5 Aberration 2 28.168 5.000 compensatingoptical element 3 2.074 2.400 1.52491 56.5 Objective 4 8.053 0.100 lens5 0.863 1.100 1.52491 56.5 6 ∞ 0.240 7 ∞ 0.100 1.61949 30.0 Transparent8 ∞ substrate Coefficients of aspherical surface First Second ThirdFourth Fifth surface surface surface surface surface κ −5.4091E+001.2966E+00 −1.2955E−01 4.7554E+01 −7.1425E−01 A₄ −1.1312E−05 −4.2412E−07−3.7832E−03 1.3641E−02 1.3647E−01 A₆ 5.1667E−04 −2.9201E−02 −5.3414E−02A₈ −1.1780E−03 −9.3339E−03 3.0269E−01 A₁₀ −2.0628E−04 3.3011E−02−1.6898E−01 A₁₂ 2.5941E−05 −2.2626E−02 A₁₄ 1.49175−04 A₁₆ −5.1578E−05Coefficients of diffractive surface First Second surface surface b₂−4.7281E−03 −4.6816E−03

In Example 4, by making the aberration compensating optical element ofolefin resin and forming the diffractive surfaces on the both opticalsurfaces which are on the light source side and the objective lens side,the axial chromatic aberration caused at the objective lens wascompensated. Further, the total power of each optical surface was 0 byforming each optical surface so as to have the negative refractive powerand the diffractive power having the same absolute value as therefractive power. Therefore, the emergent light flux diameter was notchanged in comparison with the incident light flux diameter. Further,the coefficient of the optical path difference function shown in Table5, was determined so that the second-order diffracted light had amaximum diffracted light amount.

FIG. 16 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 4 at the wavelength of 405±10 nm. FIG. 6 shows thatthe focal point is hardly shifted regardless of the wavelength.

The diffractive power of the aberration compensating optical element wasdetermined so that the axial chromatic aberration caused in the compoundsystem including the objective lens was over-corrected. Further, theaberration remained in the lens so that the spherical aberration causedin the compound system was over-corrected on the long wavelength sideand was under-corrected on the short wavelength side. The shift of thebest image point, which was caused by changing the wavelength, wasminimized by crossing the graph of the spherical aberration of thereference wavelength and ones of the spherical aberrations on the longwavelength side and the short wavelength side. Therefore, the defocuscomponent of the wavefront aberration, which was caused by the modehopping of +1 nm, could be decreased to 0.002λrms (calculated value).

In case that the chromatic aberration caused in the compound system iscompensated as described above, the interval of the diffractingring-shaped zone on the aberration compensating optical element can beexpanded in comparison with the case that the shift of the best imagepoint, which is caused by changing the wavelength, is minimized byalmost perfectly compensating the axial chromatic aberration of thecompound system and by almost perfectly compensating the sphericalaberration on the long wavelength side and the spherical aberration onthe short wavelength side. Therefore, the light amount loss caused bythe form error of the diffracting ring-shaped zone structure in themanufacture thereof can be minimized.

Example 5

In Table 6, the data relating to the optical system for the opticalpickup device according to Example 5, are shown. In FIG. 17, the opticalpath diagram of the optical system for the optical pickup deviceaccording to Example 5, is shown.

TABLE 6 Surface No. r (mm) d (mm) N_(λ) ν_(d) Note 0 ∞ Light source 1 ∞1.000 1.52491 56.5 Aberration 2 14.165 5.000 compensating opticalelement 3 2.074 2.400 1.52491 56.5 Objective 4 8.053 0.100 lens 5 0.8631.100 1.52491 56.5 6 ∞ 0.240 7 ∞ 0.100 1.61949 30.0 Transparent 8 ∞substrate Coefficients of aspherical surface Second Third Fourth Fifthsurface surface surface surface κ −1.0003E+00 −1.2955E−01 4.7554E+01−7.1425E−01 A₄ −1.0589E−04 −3.7832E−03 1.3641E−02 1.3647E−01 A₆5.1667E−04 −2.9201E−02 −5.3414E−02 A₈ −1.1780E−03 −9.3339E−03 3.0269E−01A₁₀ −2.0628E−04 3.3011E−02 −1.6898E−01 A₁₂ 2.5941E−05 −2.2626E−02 A₁₄1.4917E−04 A₁₆ −5.1578E−05 Coefficients of diffractive surface Firstsurface b₂ −9.0435E−03 b₄ 3.1856E−05

In Example 5, by making the aberration compensating optical element ofolefin resin and forming a diffractive surface which is amacroscopically plane surface, on the optical surface of the lightsource side, that is, preparing the diffractive structure on the opticalsurface of the light source side so that the diffractive structurecomprises a plurality of ring-shaped zone steps having the form of thesubstantially concentric circles which are formed on a plane surface,the axial chromatic aberration caused at the objective lens wascompensated. Further, the power of the whole system in the aberrationcompensating optical element was 0 by forming the optical surface of theobjective lens side so as to have the negative refractive power and byforming the optical surface of the light source side so as to have thediffractive power having the same absolute value as the refractivepower. Further, the coefficient of the optical path difference functionshown in Table 2, was determined so that the second-order diffractedlight had a maximum diffracted light amount.

FIG. 18 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 5 at the wavelength of 405±10 nm. FIG. 18 showsthat the focal point is hardly shifted regardless of the wavelength.

Example 6

In Table 7, the data relating to the optical system for the opticalpickup device according to Example 6, are shown. In FIG. 19, the opticalpath diagram of the optical system for the optical pickup deviceaccording to Example 6, is shown.

TABLE 7 Surface No. r (mm) d (mm) N_(λ) ν_(d) Note 0 ∞ Light source 1−17.495 1.000 1.52491 56.5 Aberration 2 ∞ 5.000 compensating opticalelement 3 2.074 2.400 1.52491 56.5 Objective 4 8.053 0.100 lens 5 0.8631.100 1.52491 56.5 6 ∞ 0.240 7 ∞ 0.100 1.61949 30.0 Transparent 8 ∞substrate Coefficients of aspherical surface First Third Fourth Fifthsurface surface surface surface κ 2.7605E+01 −1.2955E−01 4.7554E+01−7.1425E−01 A₄ 1.9423E−03 −3.7832E−03 1.3641E−02 1.3647E−01 A₆−1.4942E−03 5.1667E−04 −2.9201E−02 −5.3414E−02 A₈ −1.1780E−03−9.3339E−03 3.0269E−01 A₁₀ −2.0628E−04 3.3011E−02 −1.6898E−01 A₁₂2.5941E−05 −2.2626E−02 A₁₄ 1.4917E−04 A₁₆ −5.1578E−05 Coefficients ofdiffractive surface First surface b₂ −7.5000E−03 b₄ 3.3173E−04 b₆−4.0384E−04 b₈ 3.8224E−06 b₁₀ −1.0509E−06

In Example 6, by making the aberration compensating optical element ofolefin resin and forming the diffractive surfaces on the opticalsurfaces of the light source side, the axial chromatic aberration causedat the objective lens was compensated. Further, the power of the wholesystem in the aberration compensating optical element was 0 by formingthe optical surface of the light source side so as to have the negativerefractive power and the diffractive power having the same absolutevalue as the refractive power, and by forming the optical surface of thelight source side so as to be a plane surface. Further, the coefficientof the optical path difference function shown in Table 7, was determinedso that the second-order diffracted light had a maximum diffracted lightamount. In this example, the terms of the optical path differencefunction, to which the second or higher order coefficients were given,were used. The diffractive surface had a spherical aberration propertyto shift the spherical aberration in an under-corrected direction whenthe wavelength was shifted to the long wavelength side. Then, the changein the spherical aberration, which was caused at the objective lens whenthe wavelength was shifted to the long wavelength side, was cancelled bythe spherical aberration of the diffractive surface.

FIG. 20 is a graph showing the spherical aberration and the axialchromatic aberration of the optical system for the optical pickup deviceaccording to Example 6 at the wavelength of 405±10 nm. FIG. 20 showsthat the focal point is hardly shifted regardless of the wavelength.

Example 7

Table 8, the data relating to the optical system according to Example 7,are shown. In FIG. 21, the optical path diagram thereof is shown. FIG.22 shows the spherical aberration and the axial chromatic aberration ofthe optical system according to Example 7 at the wavelength of 405±1 nm.

TABLE 8 Surface No. r (mm) d(mm) N₄₀₄ N₄₀₅ N₄₀₆ Note 0 ∞ Light source 1−27.620 1.500 1.52506 1.52491 1.52476 Aberration 2 28.168 5.000compensating optical element 3 1.959 2.450 1.52485 1.52469 1.52454Objective 4 10.207 0.050 lens 5 0.892 1.100 1.52485 1.52469 1.52454 6 ∞0.201 7 ∞ 0.100 1.61990 1.61950 1.61909 Transparent 8 ∞ substrateCoefficients of aspherical surface First Second Third Fourth Fifthsurface surface surface surface surface κ −5.4091E+00 1.2966E+00−1.6847E−01 8.3955E+01 −7.4973E−01 A₄ −1.1312E−05 −4.2412E−07−4.8527E−03 5.4087E−03 1.2350E−01 A₆ −3.6816E−04 −6.1028E−03 1.6589E−02A₈ −3.6905E−04 −1.4958E−02 1.8042E−01 A₁₀ −2.1477E−04 1.9993E−02−1.0532E−01 A₁₂ −6.5049E−05 −1.7848E−02 A₁₄ 1.3300E−04 A₁₆ −4.2576E−05Coefficients of diffractive surface First Second surface surface b₂−4.7281E−03 −4.6816E−03

In this example, the aberration compensating optical element which is aplastic lens, is provided on the light source side of the plasticobjective lens shown in Table 4. The aberration compensating opticalelement according to this example is the same as one according toExample 4.

Example 8

In Table 9, the data relating to the optical system according to Example8, are shown. In FIG. 23, the optical path diagram thereof is shown.FIG. 24 shows the spherical aberration and the axial chromaticaberration of the optical system according to Example 8 at thewavelength of 405±1 nm.

TABLE 9 Surface No. r (mm) d (mm) N₄₀₄ N₄₀₅ N₄₀₆ Note 0 ∞ Light source 1∞ 1.000 1.52506 1.52491 1.52476 Aberration 2 14.165 5.000 compensatingoptical element 3 1.959 2.450 1.52485 1.52469 1.52454 Objective lens 410.207 0.050 5 0.892 1.100 1.52485 1.52469 1.52454 6 ∞ 0.201 7 ∞ 0.1001.61990 1.61950 1.61909 Transparent 8 ∞ substrate Coefficients ofaspherical surface Second Third Fourth Fifth surface surface surfacesurface κ −1.0003E+00 −1.6847E−01 8.3955E+01 −7.4973E−01 A₄ −1.0589E−04−4.8527E−03 5.4087E−03 1.2350E−01 A₆ −3.6816E−04 −6.1028E−03 1.6589E−02A₈ −3.6905E−04 −1.4958E−02 1.8042E−01 A₁₀ −2.1477E−04 1.9993E−02−1.0532E−01 A₁₂ −6.5049E−05 −1.7848E−02 A₁₄ 1.3300E−04 A₁₆ −4.2576E−05Coefficients of diffractive surface First surface b₂ −9.0435E−03 b₄3.1856E−05

In this example, the aberration compensating optical element which is aplastic lens, is provided on the light source side of the plasticobjective lens shown in Table 4. The aberration compensating opticalelement according to this example is the same as one according toExample 5.

In the lens data shown in Tables 5, 6 and 7, r(mm) is the radius ofcurvature, d(mm) is the surface interval, N_(λ) is the refractive indexat the wavelength of 405 nm, and ν_(d) is the Abbe constant. The designreference wavelength of the aberration compensating optical elementaccording to Examples 4 to 6 and that of the objective lens according toExamples 4 to 6 are 405 nm. Each design reference temperature thereof is25° C.

In the lens data shown in Tables 4, 8 and 9, r(mm) is the radius ofcurvature, d(mm) is the surface interval and N₄₀₄, N₄₀₅ and N₄₀₆ are therefractive indexes at the wavelength of 404 nm, 405 nm and 406 nm,respectively. The design reference wavelength of the aberrationcompensating optical element according to Examples 7 and 8 and that ofthe objective lens according to Examples 7 and 8 are 405 nm. Each designreference temperature thereof is 25° C.

In the aberration compensating optical element according to Examples 4to 8, the diffractive power (the sign thereof is positive) as adiffractive lens function and the refractive power (the sign thereof isnegative) as a refraction lens function have different sings from eachother and the same absolute values. Therefore, the paraxial power of thewhole system of the aberration compensating optical element is 0 in thedesign reference wavelength of 405 nm. Because the refractive power ofthe aberration compensating optical element is negative, the convergedlight flux is emerged from the aberration compensating optical elementwhen the temperature rises. Thereby, the third-order sphericalaberration changed in an under-corrected direction at the objective lenswhen the temperature rises, is cancelled.

FIG. 25 shows the result that the axial chromatic aberration caused atthe objective lens shown in Table 4 due to the instant wavelength changeof the light emitted from the light source is compensated by theaberration compensating optical elements according to Examples 7 and 8.When the wavelength change of ±1 nm corresponding to the wavelengthchange caused by the mode hopping of the blue-violet semiconductorleaser, occurs, the RMS value of the wavefront aberration becomes notless than 0.03λms in case of the objective lens only (”withoutaberration compensating optical element” shown in FIG. 25). However, byusing the aberration compensating optical element with the objectivelens, in both cases of Examples 7 and 8, with respect to the wavelengthchange of ±1 nm, the RMS value of the wavefront aberration becomes notmore than 0.03λms.

In FIG. 25, the wavelength change 0 corresponds to the design referencewavelength of 405 nm. When the RMS value of the wavefront aberration iscalculated, the objective lens is fixed to the best focus position forthe wavelength of 405 nm.

FIG. 26 shows the result that the third-order spherical aberrationchanged by the temperature change at the objective lens shown in Table 4is compensated by the aberration compensating optical elements accordingto Examples 7 and 8. At the temperature of 85° C. which is an operationguarantee temperature of an optical disk player, the RMS value of thewavefront aberration becomes not less than 0.03λms (maximum permissiblevalue of the wavefront aberration) in case of the objective lens only(“without aberration compensating optical element” shown in FIG. 26).However, by using the aberration compensating optical element with theobjective lens, in both cases of Examples 7 and 8, the RMS value of thewavefront aberration becomes not more than 0.03λms.

In FIG. 26, when the RMS value of the wavefront aberration iscalculated, only the change in the refractive index of the plastic lens,which is caused by the environment temperature, is considered. An amountof the change therein is about −1.0×10⁻⁴/° C.

In the above tables or the drawings, there are some cases that the powerexponent of 10 is expressed by using “E” (or “e”), such as E-02 (=10⁻²).

According to the present invention, it is possible to provide anaberration compensating optical element which can compensate thespherical aberration and the sine condition because of a large numericalaperture on the optical information recording medium side, and cancompensate the axial chromatic aberration when an objective lens inwhich the axial chromatic aberration remains, is used in order todownsize an optical device, to thin it, to lighten it, and to decreasethe cost thereof.

Further, it is possible to provide an optical system for optical pickupdevice, which can compensate the axial chromatic aberration with arelatively simple structure even though a light source having a badmonochromaticity or a light source in which a wavelength of the lightsuddenly changes, is used in a high density storage optical pickupsystem or the like, and which can be manufactured in a low cost; anoptical pickup device comprising the above optical system; and arecorder and a reproducer, comprising the above optical pickup device.

According to the present invention, it is possible to provide an opticalsystem used in optical pickup device, for suppressing the change in thespherical aberration, which is caused at an objective lens by thetemperature change, with a relatively simple structure, even though ahigh NA plastic objective lens is used in a high density storage opticalpickup system or the like; an optical pickup device comprising the aboveoptical system; and a recorder and a reproducer, comprising the aboveoptical pickup device.

Further, it is possible to provide an aberration compensating opticalelement which can compensate the change in the spherical aberration,which is caused at an objective lens by the temperature change, when aplastic objective lens in which the change of the spherical aberrationis large because of the temperature change, is used.

Further, it is possible to provide an optical system for optical pickupdevice, which can compensate the axial chromatic aberration with arelatively simple structure even though a light source having a badmonochromaticity or a light source in which a wavelength of the lightsuddenly changes, is used in a high density storage optical pickupsystem or the like, and which can be manufactured in a low cost; anoptical pickup device comprising the above optical system; and arecorder and a reproducer, comprising the above optical pickup device.

Further, it is possible to provide an aberration compensating opticalelement which can compensate the spherical aberration and the sinecondition because of a large numerical aperture on the opticalinformation recording medium side, and can compensate the axialchromatic aberration when an objective lens in which the axial chromaticaberration remains, is used in order to downsize an optical device, tothin it, to lighten it, and to decrease the cost thereof.

The entire disclosure of Japanese Patent Applications No. Tokugan2001-210659 filed on Jul. 11, 2001, No. Tokugan 2001-284407 filed onSep. 19, 2001 and No. Tokugan 2002-53085 filed on Feb. 28, 2002including specification, claims drawings and summary are incorporatedherein by reference in its entirety.

What is claimed is:
 1. An aberration compensating optical elementcomprising: a diffractive structure having a plurality of ring-shapedzone steps formed on at least one surface of the aberration compensatingoptical element; wherein the aberration compensating optical element isadapted for being disposed on an optical path between a light source foremitting a light having a wavelength of not more than 550 nm, and anobjective lens made of a material having an Abbe constant of not morethan 95.0 at a d-line; and wherein the following inequality issatisfied: P_(λ1)<P_(λ0)<P_(λ2), where P_(λ0) is a paraxial power (mm⁻¹)of the aberration compensating optical element at the wavelength λ₀ ofthe light emitted from the light source; P_(λ1) is a paraxial power(mm⁻¹) of the aberration compensating optical element at a wavelength λ₁which is 10 nm shorter than the wavelength λ₀; and P_(λ2) is a paraxialpower (mm⁻¹) of the aberration compensating optical element at awavelength λ₂ which is 10 nm longer than the wavelength λ₀.
 2. Anaberration compensating optical element comprising: a diffractivestructure having a plurality of ring-shaped zone steps formed on atleast one surface of the aberration compensating optical element;wherein the aberration compensating optical element is adapted for beingdisposed on an optical path between a light source for emitting a lighthaving a wavelength of not more than 550 nm, and an objective lens madeof a material having an Abbe constant of not more than 95.0 at a d-line;and wherein at least one ring-shaped zone step having a step distance Δ(mm) in a direction of an optical axis between adjacent steps of theplurality of ring-shaped zone steps is formed within an effectivediameter so that m, defined by following equations: m=INT(Y),Y=Δ×(n−1)/(λ₀×10⁻³),  is an integer except 0 and ±1, where INT(Y) is aninteger obtained by rounding Y, λ₀ is the wavelength (nm) of the lightemitted from the light source, and n is a refractive index of theaberration compensating optical element at the wavelength λ₀ (nm).
 3. Anaberration compensating optical element comprising: a single lens;wherein the single lens has one optical surface having a diffractivestructure having a plurality of ring-shaped zone steps formed on a planesurface and another optical surface opposite to the one optical surface,which has a concave refractive surface; and wherein the aberrationcompensating optical element is adapted for being disposed on an opticalpath between a light source for emitting a light having a wavelength ofnot more than 550 nm, and an objective lens made of a material having anAbbe constant of not more than 95.0 at a d-line.
 4. The aberrationcompensating optical element of claim 1; wherein the followinginequality is satisfied: 0.5×10⁻² <P _(D)<15.0×10⁻², where P_(D) is aparaxial power (mm⁻¹) of the diffractive structure and is defined by thefollowing equation: P _(D)=Σ(−2·b _(2i) ·ni), when an optical pathdifference function is defined by the following equation: Φbi=ni·(b_(2i) ·hi ² +b _(4i) ·hi ⁴ +b _(6i) ·hi ⁶+ . . . ), as a function thatan optical path difference Φbi added to a wavefront transmitting throughthe aberration compensating optical element, by the diffractivestructure formed on an i-th surface of the aberration compensatingoptical element, is expressed by using a height hi (mm) from the opticalaxis; where ni is a diffraction order of a diffracted light having amaximum diffracted light amount among a plurality of diffracted lightsgenerated by the diffractive structure formed on the i-th surface, andb_(2i), b_(4i), b_(6i), . . . are a second order coefficient of theoptical path difference function, a fourth order one, a sixth order one. . . , respectively.
 5. The aberration compensating optical element ofclaim 4, wherein the following inequality is satisfied: 1.0×10⁻² <P_(D)<10.0×10⁻².
 6. An aberration compensating optical elementcomprising: a diffractive structure having a plurality of ring-shapedzone steps formed on at least one surface of the aberration compensatingoptical element; wherein the following inequality is satisfied: 1.0×10⁻²<P _(D)<10.0×10⁻², where P_(D) is a paraxial power (mm⁻¹) of thediffractive structure and is defined by the following equation: P_(D)=Σ(−2·b _(2i) ·ni), when an optical path difference function isdefined by the following equation: Φbi=ni·(b _(2i) ·hi ² +b _(4i) ·hi ⁴+b _(6i) ·hi ⁶+ . . . ), as a function that an optical path differenceΦbi added to a wavefront transmitting through the aberrationcompensating optical element, by the diffractive structure formed on ani-th surface of the aberration compensating optical element, isexpressed by using a height hi (mm) from an optical axis; where ni is adiffraction order of a diffracted light having a maximum diffractedlight amount among a plurality of diffracted lights generated by thediffractive structure formed on the i-th surface, and b_(2i), b_(4i),b_(6i), . . . are a second order coefficient of the optical pathdifference function, a fourth order one, a sixth order one . . . ,respectively.
 7. The aberration compensating optical element of claim 1,wherein the paraxial power P_(λ0) of the aberration compensating opticalelement is substantially zero at the wavelength λ₀ of the light emittedfrom the light source.
 8. The aberration compensating optical element ofclaim 7, wherein the following inequalities are satisfied: P_(D)>0P_(R)<0 −0.9<P _(D) /P _(R)<−1.1, where P_(D) is a paraxial power (mm⁻¹)of the diffractive structure and is defined by the following equation: P_(D)=Σ(−2·b _(2i) ·ni), when an optical path difference function isdefined by the following equation: Φb=ni·(b _(2i) ·hi ² +b _(4i) ·hi ⁴+b _(6i) ·hi ⁶+ . . . ), as a function that an optical path differenceΦbi added to a wavefront transmitting through the aberrationcompensating optical element, by the diffractive structure formed on ani-th surface of the aberration compensating optical element, isexpressed by using a height hi (mm) from an optical axis; where ni is adiffraction order of a diffracted light having a maximum diffractedlight amount among a plurality of diffracted lights generated by thediffractive structure formed on the i-th surface, and b_(2i), b_(4i),b_(6i), . . . are a second order coefficient of the optical pathdifference function, a fourth order one, a sixth order one . . . ,respectively: and P_(R) is a refractive power (mm⁻¹) of the aberrationcompensating optical element as a refractive lens.
 9. The aberrationcompensating optical element of claim 1, wherein the diffractivestructure has such a spherical aberration property that a sphericalaberration of an emergent light flux is changed in an under-correcteddirection or an over-corrected direction when a wavelength of anincident light flux is shifted to a longer wavelength side; wherein thediffractive structure is formed so as to satisfy the followinginequality: 0.2≦|(P _(hf) /P _(hm))−2|≦6.0, where P_(hf) is a firstinterval in a direction to perpendicular to an optical axis of thediffractive structure between adjacent steps of the ring-shaped zones ofthe diffractive structure at a diameter hf which is a half of a maximumeffective diameter hm, and P_(hm) is a second interval in the directionto perpendicular to the optical axis of the diffractive structurebetween adjacent steps of the ring-shaped zones of the diffractivestructure at the maximum effective diameter hm.
 10. The aberrationcompensating optical element of claim 1, wherein when a wavelength of alight entering the diffractive structure is not more than 550 nm, adiffraction efficiency of the diffractive structure becomes maximal. 11.The aberration compensating optical element of claim 1; wherein theaberration compensating optical element is a plastic lens.
 12. Anoptical system for carrying out at least one of a record of informationon an information recording surface of an optical information recordingmedium and a reproduction of information from the information recordingsurface; comprising: a light source for emitting a light having awavelength of not more than 550 nm; an objective lens made of a materialhaving an Abbe constant of not more than 95.0 at a d-line; and theaberration compensating optical element of claim 1, which is disposed onan optical path between the light source and the objective lens.
 13. Anoptical pickup device for carrying out at least one of a record ofinformation on an information recording surface of an opticalinformation recording medium and a reproduction of information from theinformation recording surface; comprising: a condensing optical systemhaving the optical system of claim
 12. 14. A recorder for recoding atleast one of a sound and an image, comprising the optical pickup deviceof claim
 13. 15. A reproducer for reproducing at least one of a soundand an image, comprising the optical pickup device of claim
 13. 16. Anaberration compensating optical element comprising: a plastic lenshaving a single lens structure, and comprising a diffractive structurehaving a plurality of ring-shaped zone steps formed on at least onesurface of the plastic lens; wherein the aberration compensating opticalelement is adapted for being disposed on an optical path between a lightsource and an objective lens having an image-side numerical aperture ofnot less than 0.75 and comprising at least one plastic lens; and whereinthe aberration compensating optical element decreases a changeΔ3SA_(OBJ) in a third-order spherical aberration of the objective lens,which is caused by a refractive index change ΔN_(OBJ) of at least oneplastic lens contained in the objective lens due to a temperature changeof the objective lens, by using an inclination change of a marginal rayof an emergent light flux from the aberration compensating opticalelement, which is caused by a refractive index change ΔN_(AC) of theaberration compensating optical element due to a temperature change ofthe aberration compensating optical element.
 17. The aberrationcompensating optical element of claim 16, wherein the followinginequality is satisfied: P_(T1)<P_(T0)<P_(T2), where P_(T0) is aparaxial power (mm⁻¹) of the aberration compensating optical element ata predetermined temperature T₀; P_(T1) is a paraxial power (mm⁻¹) of theaberration compensating optical element at a temperature T₁ which islower than the predetermined temperature T₀; and P_(T2) is a paraxialpower (mm⁻¹) of the aberration compensating optical element at atemperature T₂ which is higher than the predetermined temperature T₀.18. The aberration compensating optical element of claim 17, wherein theobjective lens is one having a doublet lens structure in which a firstlens having a positive refractive power and a second lens having apositive refractive power are arranged in an order from a side of theobjective lens; and wherein at least the first lens is a plastic lens.19. The aberration compensating optical element of claim 18, wherein thefollowing inequalities are satisfied: P_(R)<0 0<ΔP _(AC) /ΔT_(AC)<1×10⁻⁴, where P_(R) is a refractive power (mm⁻¹) of the aberrationcompensating optical element as a refractive lens; and ΔP_(AC) is anamount of a change in a paraxial power (mm⁻¹) of the aberrationcompensating optical element, which is caused by the temperature changeΔT_(AC) (° C.) of the aberration compensating optical element.
 20. Theaberration compensating optical element of claim 18, wherein the lightsource is one for emitting a light having a wavelength of not more than550 nm; and wherein the following inequality is satisfied:P_(λ1)<P_(λ0)<P_(λ2), where P_(λ0) is a paraxial power (mm⁻¹) of theaberration compensating optical element at the wavelength λ₀ of thelight emitted from the light source; P_(λ1) is a paraxial power (mm⁻¹)of the aberration compensating optical element at a wavelength λ₁ whichis 10 nm shorter than the wavelength λ₀; and P_(λ2) is a paraxial power(mm⁻¹) of the aberration compensating optical element at a wavelength λ₂which is 10 nm longer than the wavelength λ₀.
 21. The aberrationcompensating optical element of claim 18, wherein the followinginequities are satisfied: |Δ3SA _(OBJ)|/|Δ5SA _(OBJ)|>1, −30.0×10⁻⁴<Δ3SA_(OBJ)/(ΔT _(OBJ) ·NA ⁴ ·f _(OBJ))<0, 3×10⁻² <ΔfB _(OBJ) ·νd _(OBJ) /f_(OBJ)<14×10⁻², 1.0×10⁻² <P _(D)<10.0×10⁻², where Δ3SA_(OBJ) is a changein a third-order spherical aberration component of a Zernike polynomialinto which an aberration of the objective lens is expanded, in case thata refractive index of the plastic lens in the objective lens is changedby ΔN_(OBJ) due to the temperature change ΔT_(OBJ) (° C.) of theobjective lens; the change in the third-order spherical aberration beingexpressed by a root mean square value by a wavelength λ₀ of a lightemitted from the light source; and a sign of the change in thethird-order spherical aberration being positive when the third-orderspherical aberration component is changed in an over-correcteddirection, and being negative when the third-order spherical aberrationcomponent is changed in an under-corrected direction; Δ5SA_(OBJ) is achange in a fifth-order spherical aberration component of the Zernikepolynomial into which the aberration of the objective lens is expanded,in case that the refractive index of the plastic lens in the objectivelens is changed by ΔN_(OBJ) due to the temperature change ΔT_(OBJ) (°C.) of the objective lens; the change in the fifth-order sphericalaberration being expressed by a root mean square value by the wavelengthλ₀ of a light emitted from the light source; NA is a predeterminedimage-side numerical aperture which is required for at least one of arecord of information on an optical information recording medium and areproduction of information from the optical information recordingmedium; f_(OBJ) is a focal length (mm) of the objective lens; ΔfB_(OBJ)is an axial chromatic aberration (mm) occurring at the objective lenswhen a light having a wavelength which is +10 nm longer than thewavelength λ₀ of a light emitted from the light source enters theobjective lens; νd_(OBJ) is a mean value of an Abbe constant of thefirst lens in the objective lens at the d-line and an Abbe constant ofthe second lens at the d-line; and P_(D) is a paraxial power (mm⁻¹) ofthe diffractive structure and is defined by the following equation: P_(D)=Σ(−2·b _(2i) ·ni), when an optical path difference function isdefined by the following equation: Φbi=ni·(b _(2i) ·hi ² +b _(4i) 12 ·hi^(4i) +b _(6i) ·hi ⁶+ . . . ), as a function that an optical pathdifference Φbi added to a wavefront transmitting through the diffractivestructure formed on an i-th surface of the aberration compensatingoptical element, by the diffractive structure formed on the i-thsurface, is expressed by using a height hi (mm) from an optical axis;where b_(2i), b_(4i), b_(6i), . . . are a second order coefficient ofthe optical path difference function, a fourth order one, a sixth orderone . . . , respectively, and ni is a diffraction order of a diffractedlight having a maximum diffracted light amount among a plurality ofdiffracted lights generated by the diffractive structure formed on thei-th surface.
 22. The aberration compensating optical element of claim16, comprising one optical surface having a diffractive structure havinga plurality of ring-shaped zone steps formed on a plane surface andanother optical surface opposite to the one optical surface, which has aconcave refractive surface.
 23. The aberration compensating opticalelement of claim 16, wherein a paraxial power P_(λ0) of the aberrationcompensating optical element is substantially zero at a wavelength λ₀ ofa light emitted from the light source.
 24. The aberration compensatingoptical element of claim 23, wherein the following inequalities aresatisfied: P_(D)>0 P_(R)<0 −0.9<P _(D) /P _(R)<−1.1, where P_(D) is aparaxial power (mm⁻¹) of the diffractive structure and is defined by thefollowing equation: P _(D)=Σ(−2·b _(2i) ·ni), when an optical pathdifference function is defined by the following equation: Φbi=ni·(b_(2i) ·hi ² +b _(4i) ·hi ⁴ +b _(6i) ·hi ⁶+ . . . ), as a function thatan optical path difference Φbi added to a wavefront transmitting throughthe diffractive structure formed on an i-th surface of the aberrationcompensating optical element, by the diffractive structure formed on thei-th surface, is expressed by using a height hi (mm) from an opticalaxis; where b_(2i), b_(4i), b_(6i), . . . are a second order coefficientof the optical path difference function, a fourth order one, a sixthorder one . . . , respectively, and ni is a diffraction order of adiffracted light having a maximum diffracted light amount among aplurality of diffracted lights generated on the diffractive structureformed on the i-th surface; and P_(R) is a refractive power (mm⁻¹) ofthe aberration compensating optical element as a refractive lens. 25.The aberration compensating optical element of claim 16, wherein thefollowing inequality is satisfied: P_(T2)<P_(T0)<P_(T1), where P_(T0) isa paraxial power (mm⁻¹) of the aberration compensating optical elementat a predetermined temperature T₀; P_(T1) is a paraxial power (mm⁻¹) ofthe aberration compensating optical element at a temperature T₂ which islower than the predetermined temperature T₀; and P_(T2) is a paraxialpower (mm⁻¹) of the aberration compensating optical element at atemperature T₂ which is higher than the predetermined temperature T₀.26. The aberration compensating optical element of claim 25, wherein theobjective lens is a plastic lens having a single lens structure.
 27. Theaberration compensating optical element of claim 16, wherein at leastone ring-shaped zone step having a step distance Δ (mm) in a directionof an optical axis between adjacent steps of the plurality ofring-shaped zone steps is formed within an effective diameter so that m,defined by following equations: m=INT(Y), Y=Δ×(n−1)/(λ₀×10⁻³), is aninteger except 0 and ±1, where INT(Y) is an integer obtained by roundingY, λ₀ is the wavelength (nm) of the light emitted from the light source,and n is a refractive index of the aberration compensating opticalelement at the wavelength λ₀ (nm).
 28. The aberration compensatingoptical element of claim 16, comprising a plurality of diffractivestructures having a plurality of ring-shaped zone steps formed on bothsurfaces of the plastic lens.
 29. The aberration compensating opticalelement of claim 16, wherein the diffractive structure has such aspherical aberration property that a spherical aberration of an emergentlight flux is changed in an under-corrected direction or anover-corrected direction when a wavelength of an incident light flux isshifted to a longer wavelength side; wherein the diffractive structureis formed so as to satisfy the following inequality: 0.2≦|(P _(hf) /P_(hm))−2|≦6.0, where P_(hf) is a first interval in a direction toperpendicular to an optical axis of the diffractive structure betweenadjacent steps of the ring-shaped zones of the diffractive structure ata diameter hf which is a half of a maximum effective diameter hm, andP_(hm) is a second interval in the direction to perpendicular to theoptical axis of the diffractive structure between adjacent steps of thering-shaped zones of the diffractive structure at the maximum effectivediameter hm.
 30. The aberration compensating optical element of claim16, wherein when a wavelength of a light entering the diffractivestructure is not more than 550 nm, a diffraction efficiency of thediffractive structure becomes maximal.
 31. An optical system forcarrying out at least one of a record of information on an informationrecording surface of an optical information recording medium and areproduction of information from the information recording surface;comprising: a light source; an objective lens having an image-sidenumerical aperture of not less than 0.75 and comprising at least oneplastic lens; and the aberration compensating optical element of claim16, which is disposed on an optical path between the light source andthe objective lens.
 32. An optical pickup device for carrying out atleast one of a record of information on an information recording surfaceof an optical information recording medium and a reproduction ofinformation from the information recording surface; comprising: acondensing optical system having the optical system of claim
 31. 33. Arecorder for recoding at least one of a sound and an image, comprisingthe optical pickup device of claim
 32. 34. A reproducer for reproducingat least one of a sound and an image, comprising the optical pickupdevice of claim 32.